Fair Value of Assets and Liabilities (Changes in Level 3 Assets and Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	$ 10,617	$ 5,441	$ 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	(1,008)	0	(1,008)	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	1	147	1
Fair Value, end of period	9,309	5,854	9,309	5,854
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	(300)	412	412	638
Net unrealized investment gains (losses):
Included in earnings	(462)	61	250	287
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	11	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	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	(9)	0	(31)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	(1)	0
Fair Value, end of period	0	450	0	450
Short-term investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(2)	0	(55)	0
Net unrealized investment gains (losses):
Included in earnings	(2)	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		0		0
Fair Value, end of period		375		375
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	195,497	218,531	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	4,662	4,911	9,513	9,784
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	795	3,834	(11,523)	(2,483)
Fair Value, end of period	185,258	268,337	185,258	268,337
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(15,696)	41,061	(56,738)	20,945
Net unrealized investment gains (losses):
Included in earnings	(14,217)	43,088	(52,511)	24,869
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	34,163	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	(34,163)	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	6,578,210	(7,169,218)	(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	(256,229)	(248,909)	(511,149)	(495,258)
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	(6,222,612)	(9,331,040)	(6,222,612)	(9,331,040)
Future policy benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	611,827	(1,912,913)	2,440,439	(1,128,449)
Net unrealized investment gains (losses):
Included in earnings	537,900	(1,972,296)	2,272,290	(1,246,436)
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
Reinsurance payables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	129
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0
Net investment income	0
Purchases	0
Sales	0
Issuances	0
Settlements	0
Transfers into Level 3	0
Transfers out of Level 3	0
Other	129
Fair Value, end of period	0		0
Reinsurance payables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Reinsurance payables | Asset administration fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Policyholders' Account Balances
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		0
Net investment income	0		0
Purchases	0		0
Sales	0		0
Issuances	(13,790)		(13,790)
Settlements	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	0		0
Fair Value, end of period	13,835		13,835
Policyholders' Account Balances | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(45)		(45)
Net unrealized investment gains (losses):
Included in earnings	(45)		(45)
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	5,882	980	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	776	2,170	1,421	2,176
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	1	0	975
Fair Value, end of period	6,658	3,151	6,658	3,151
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	90,273	140,837	95,206	151,989
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(2,563)	1,168	(1,368)	651
Net investment income	64	1,482	114	4,857
Purchases	4,874	604	5,025	1,094
Sales	(205)	(341)	(241)	(14,367)
Issuances	0	0	0	0
Settlements	(2,369)	(5,734)	(8,404)	(6,293)
Transfers into Level 3	6,941	935	6,941	2,350
Transfers out of Level 3	0	(3,248)	(215)	(3,625)
Other	0	0	23	(974)
Fair Value, end of period	96,849	133,567	96,849	133,567
Available-for-sale | Fixed maturities | Corporate securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(166)	(2,136)	(232)	(2,115)
Net unrealized investment gains (losses):
Included in earnings	(144)	(2,194)	(221)	(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	256,244	148,869	185,358	31,735
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(439)	111	(584)	271
Net investment income	(12)	28	33	52
Purchases	21,000	99,159	84,810	150,607
Sales	(193)	(1,595)	(193)	(1,595)
Issuances	0	0	0	0
Settlements	(14,714)	(42,510)	(23,613)	(42,552)
Transfers into Level 3	10,025	26,921	51,979	92,466
Transfers out of Level 3	(212,780)	(15,489)	(238,659)	(15,489)
Other	0	0	0	0
Fair Value, end of period	59,178	215,503	59,178	215,503
Available-for-sale | Fixed maturities | Structured securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	47	9	47	8
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