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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	$ 9,309	$ 5,854	$ 9,758	$ 4,864
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	351
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	(1,584)	0	(2,592)	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	0	3,853	147	3,854
Fair Value, end of period	7,679	10,337	7,679	10,337
Equity 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
Equity Securities | Asset administration fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	(46)	630	366	1,268
Net unrealized investment gains (losses):
Included in earnings	(375)	630	(125)	917
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	450	87	450
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	0	75	0	75
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	31	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	(23)	(450)	(24)	(450)
Fair Value, end of period	8	75	8	75
Short-term investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	(55)	0
Net unrealized investment gains (losses):
Included in earnings	0	0	(55)	0
Short-term investments | Asset administration 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
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		0
Other		(375)		(375)
Fair Value, end of period		0		0
Cash equivalents | 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
Cash equivalents | Asset administration fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings		0		0
Net unrealized investment gains (losses):
Included in earnings		0		0
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0		147
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		0
Other	0		(147)
Fair Value, end of period	0		0
Other invested assets | 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
Other invested assets | Asset administration fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Reinsurance Recoverables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	185,258	268,337	244,006	240,091
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	4,930	4,875	14,444	14,659
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	2,485	2,948	(9,039)	465
Fair Value, end of period	159,206	242,381	159,206	242,381
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(33,467)	(33,779)	(90,205)	(12,834)
Net unrealized investment gains (losses):
Included in earnings	(47,218)	(31,836)	(95,836)	(7,021)
Reinsurance Recoverables | Asset administration 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	0	0	33,962
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	(106)	0
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	34,269	0
Transfers out of Level 3	0	0	(34,163)	(33,962)
Other	0	0	0	0
Fair Value, end of period	0	0	0	0
Receivables from parents and affiliates | 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
Receivables from parents and affiliates | Asset administration 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	(6,222,612)	(9,331,040)	(8,151,902)	(7,707,333)
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	(258,121)	(252,024)	(769,270)	(747,282)
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	(4,995,305)	(7,999,051)	(4,995,305)	(7,999,051)
Future policy benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	1,485,428	1,584,013	3,925,867	455,564
Net unrealized investment gains (losses):
Included in earnings	1,428,629	1,503,393	3,677,070	274,784
Future policy benefits | Asset administration 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
Policyholders' Account Balances
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(13,835)		0
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	(19,141)		(32,931)
Settlements	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	0		0
Fair Value, end of period	(34,069)		(34,069)
Policyholders' Account Balances | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(1,093)		(1,138)
Net unrealized investment gains (losses):
Included in earnings	(1,093)		(1,138)
Policyholders' Account Balances | Asset administration fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Available-for-sale | Fixed maturities | 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	6,658	3,151	5,237	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	255	917	1,676	3,093
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	0	(2)	0	973
Fair Value, end of period	6,913	4,066	6,913	4,066
Available-for-sale | Fixed maturities | 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	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | U.S. Treasury securities and obligations of U.S. government authorities and agencies | Asset administration 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
Available-for-sale | Fixed maturities | Foreign government
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
Available-for-sale | Fixed maturities | Foreign government | 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
Available-for-sale | Fixed maturities | Foreign government | Asset administration fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings		0		0
Net unrealized investment gains (losses):
Included in earnings		0		0
Available-for-sale | Fixed maturities | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	96,849	133,567	95,206	151,989
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(4,437)	(2,270)	(5,805)	(1,619)
Net investment income	45	1,526	159	6,383
Purchases	1,323	334	6,348	1,428
Sales	0	0	(241)	(14,367)
Issuances	0	0	0	0
Settlements	(6,495)	(1,670)	(14,899)	(7,963)
Transfers into Level 3	0	8,290	6,941	10,640
Transfers out of Level 3	0	(1,056)	(215)	(4,681)
Other	3	(3,028)	26	(4,002)
Fair Value, end of period	84,864	134,748	84,864	134,748
Available-for-sale | Fixed maturities | Corporate securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(2,424)	(945)	(2,656)	(3,060)
Net unrealized investment gains (losses):
Included in earnings	(2,771)	0	(2,992)	(2,277)
Available-for-sale | Fixed maturities | Corporate securities | Asset administration 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
Available-for-sale | Fixed maturities | Structured securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	59,178	215,503	185,358	31,735
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	284	(306)	(300)	(35)
Net investment income	2	111	35	163
Purchases	0	78,112	84,810	228,719
Sales	(13,850)	0	(14,043)	(1,595)
Issuances	0	0	0	0
Settlements	(14,052)	(6,720)	(37,665)	(49,272)
Transfers into Level 3	(194)	2,513	51,785	94,979
Transfers out of Level 3	(20,975)	(12,502)	(259,634)	(27,991)
Other	0	0	0	0
Fair Value, end of period	9,952	277,238	9,952	277,238
Available-for-sale | Fixed maturities | Structured securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(441)	527	(394)	535
Net unrealized investment gains (losses):
Included in earnings	0	0	0	(1)
Available-for-sale | Fixed maturities | Structured securities | Asset administration 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