Fair Value of Assets and Liabilities (Changes in Level 3 Assets and Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fixed Maturities Available-For-Sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	$ 3,151		$ 0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		0
Net investment income	0		0
Purchases	917		3,093
Sales	0		0
Issuances	0		0
Settlements	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	(2)		973
Fair Value, end of period	4,066		4,066
Fixed Maturities Available-For-Sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Fixed Maturities Available-For-Sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Fixed Maturities Available-For-Sale | Foreign government bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0		87
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		0
Net investment income	0		0
Purchases	0		0
Sales	0		0
Issuances	0		0
Settlements	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		(87)
Other	0		0
Fair Value, end of period	0		0
Fixed Maturities Available-For-Sale | Foreign government bonds | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Fixed Maturities Available-For-Sale | Foreign government bonds | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Fixed Maturities Available-For-Sale | U.S. Corporate Public Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	1,911	$ 15,000	15,598	$ 15,000
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	(15)	0
Net investment income	5	0	20	0
Purchases	0	0	0	0
Sales	0	0	(14,026)	0
Issuances	0	0	0	0
Settlements	(312)	0	(976)	0
Transfers into Level 3	0	0	2,350	0
Transfers out of Level 3	(1,056)	0	(1,433)	0
Other	0	0	(974)	0
Fair Value, end of period	559	15,000	559	15,000
Fixed Maturities Available-For-Sale | U.S. Corporate Public Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	11	0	15	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Fixed Maturities Available-For-Sale | U.S. Corporate Public Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Fixed Maturities Available-For-Sale | U.S. Corporate Private Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	122,609	126,673	124,864	107,777
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(1,659)	(661)	(1,769)	(3,548)
Net investment income	1,510	1,417	6,329	4,198
Purchases	306	1,245	1,348	12,472
Sales	0	0	(341)	0
Issuances	0	0	0	0
Settlements	(1,358)	(1,057)	(6,987)	(1,481)
Transfers into Level 3	5,322	0	5,322	10,176
Transfers out of Level 3	0	0	0	(1,015)
Other	(3,014)	0	(3,014)	0
Fair Value, end of period	122,776	127,659	122,776	127,659
Fixed Maturities Available-For-Sale | U.S. Corporate Private Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(940)	42	(2,976)	(920)
Net unrealized investment gains (losses):
Included in earnings	0	0	(2,194)	(962)
Fixed Maturities Available-For-Sale | U.S. Corporate Private Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Fixed Maturities Available-For-Sale | Foreign Corporate Public Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	0	0	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	13	1	13	1
Net investment income	0	(1)	0	(1)
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	675	129	675	129
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	688	129	688	129
Fixed Maturities Available-For-Sale | Foreign Corporate Public Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Fixed Maturities Available-For-Sale | Foreign Corporate Public Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Fixed Maturities Available-For-Sale | Foreign Corporate Private Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	9,047	14,582	11,527	4,531
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(624)	(710)	152	277
Net investment income	11	58	34	185
Purchases	28	1	80	1,990
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	(5,390)	0	(7,128)
Transfers into Level 3	2,293	0	2,293	8,686
Transfers out of Level 3	0	(490)	(3,248)	(490)
Other	(14)	0	(14)	0
Fair Value, end of period	10,725	8,928	10,725	8,928
Fixed Maturities Available-For-Sale | Foreign Corporate Private Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(16)	877	(99)	877
Net unrealized investment gains (losses):
Included in earnings	0	0	(83)	0
Fixed Maturities Available-For-Sale | Foreign Corporate Private Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Fixed Maturities Available-For-Sale | Asset-Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	215,503	87,371	31,735	46,493
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(306)	329	(35)	367
Net investment income	111	37	163	125
Purchases	78,112	7,820	228,719	61,939
Sales	0	(6,489)	(1,595)	(6,739)
Issuances	0	0	0	0
Settlements	(6,720)	(45)	(49,272)	(533)
Transfers into Level 3	2,513	7,137	94,979	24,961
Transfers out of Level 3	(12,502)	(51,690)	(27,991)	(82,145)
Other	0	0	0	0
Fair Value, end of period	277,238	44,442	277,238	44,442
Fixed Maturities Available-For-Sale | Asset-Backed Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	527	(28)	535	(26)
Net unrealized investment gains (losses):
Included in earnings	0	(26)	(1)	(26)
Fixed Maturities Available-For-Sale | Asset-Backed Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Trading account assets | Equity Securities, Available-For-Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	5,854	5,076	4,864	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	0	(1)	0	3,422
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	3,853	0	4,205	1,565
Fair Value, end of period	10,337	5,514	10,337	5,514
Trading account assets | Equity Securities, Available-For-Sale | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Trading account assets | Equity Securities, Available-For-Sale | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	630	439	1,268	527
Net unrealized investment gains (losses):
Included in earnings	630	437	917	526
Equity Securities, Available-For-Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0		0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		351
Net investment income	0		0
Purchases	0		0
Sales	0		0
Issuances	0		0
Settlements	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	0		(351)
Fair Value, end of period	0		0
Equity Securities, Available-For-Sale | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Equity Securities, Available-For-Sale | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	450	450	450	450
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	75	0	75	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	(450)	0	(450)	0
Fair Value, end of period	75	450	75	450
Short-term investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Short-term investments | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Cash equivalents
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	375	375	375	225
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	150	0	150
Purchases	0	(150)	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	(375)	0	(375)	0
Fair Value, end of period	0	375	0	375
Cash equivalents | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Cash equivalents | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Other Long- term Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0		0	1,565
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		0	0
Net investment income	0		0	0
Purchases	0		0	0
Sales	0		0	0
Issuances	0		0	0
Settlements	0		0	0
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		0	0
Other	0		0	(1,565)
Fair Value, end of period	0	0	0	0
Other Long- term Investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		0	0
Net unrealized investment gains (losses):
Included in earnings	0		0	0
Other Long- term Investments | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0	0
Net unrealized investment gains (losses):
Included in earnings	0		0	0
Reinsurance Recoverables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	268,337	345,918	240,091	3,012,653
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	4,875	4,959	14,659	65,525
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	2,948	0	465	0
Fair Value, end of period	242,381	335,189	242,381	335,189
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(33,779)	(15,688)	(12,834)	(2,742,989)
Net unrealized investment gains (losses):
Included in earnings	(31,836)	(10,472)	(7,021)	91,513
Reinsurance Recoverables | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Receivables From Parents And Affiliates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	2,776	33,962	7,664
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	19	0	88
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	24	0	(1,988)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(1)	(33,962)	(2,958)
Other	0	0	0	0
Fair Value, end of period	0	2,795	0	2,795
Receivables From Parents And Affiliates | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	(23)	0	(11)
Net unrealized investment gains (losses):
Included in earnings	0	(1,772)	0	(1,772)
Receivables From Parents And Affiliates | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Future policy benefits
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(9,331,040)	(12,326,533)	(7,707,333)	(3,134,077)
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	(252,024)	(243,012)	(747,282)	(536,984)
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other(5)	0	0	0	0
Fair Value, end of period	(7,999,051)	(12,040,828)	(7,999,051)	(12,040,828)
Future policy benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	1,584,013	528,717	455,564	(8,369,767)
Net unrealized investment gains (losses):
Included in earnings	1,503,393	460,383	274,784	(8,124,372)
Future policy benefits | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	$ 0	$ 0	$ 0	$ 0