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	$ 16,625	$ 17,350	$ 17,525	$ 15,845
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	(1)	0	(33)
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	(75)	0	(75)	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	11	0	11
Fair Value, end of period	16,327	17,162	16,327	17,162
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 management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	(223)	(198)	(1,123)	1,339
Net unrealized investment gains (losses):
Included in earnings	(222)	(198)	(1,123)	1,977
Short-term Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	333	0	1,339	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	5,440	3,319	10,710	3,319
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	(5,316)	(2,607)	(11,560)	(2,607)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	(1)	0	(15)	0
Fair Value, end of period	456	712	456	712
Short-term Investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	(18)	0
Net unrealized investment gains (losses):
Included in earnings	(1)	0	(19)	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	564		0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		0
Net investment income	0		0
Purchases	0		2,550
Sales	0		0
Issuances	0		0
Settlements	(564)		(2,550)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	0		0
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 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
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	5	0	0	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	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	9	8
Transfers out of Level 3	(9)	0	(9)	(8)
Other	0	0	0	0
Fair Value, end of period	0	0	0	0
Other invested assets | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	4	0	0	0
Net unrealized investment gains (losses):
Included in earnings	4	0	0	0
Other invested assets | 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
Reinsurance Recoverables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	3,943,881	7,192,057	5,457,649	5,474,263
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	224,620	223,330	665,401	679,236
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	(760,102)	0	(760,102)
Fair Value, end of period	2,958,093	5,277,737	2,958,093	5,277,737
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(1,210,408)	(1,377,548)	(3,164,957)	(115,660)
Net unrealized investment gains (losses):
Included in earnings	(1,174,501)	(1,320,981)	(3,000,498)	(308,066)
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	0	0	6,493
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	(20)	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	6,551	0
Transfers out of Level 3	0	0	(6,531)	(6,493)
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 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 | Interest credited to polcyholder account balances
Total gains (losses) (realized/unrealized):
Included in earnings		0		0
Net unrealized investment gains (losses):
Included in earnings		0		0
Future policy benefits
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(3,916,973)	(6,424,559)	(5,452,583)	(5,041,007)
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	(231,467)	(221,503)	(686,055)	(654,141)
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	(2,908,324)	(5,272,326)	(2,908,324)	(5,272,326)
Future policy benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	1,240,116	1,373,736	3,230,314	422,822
Net unrealized investment gains (losses):
Included in earnings	1,204,459	1,327,927	3,065,835	311,314
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
Future policy benefits | Interest credited to polcyholder account balances
Total gains (losses) (realized/unrealized):
Included in earnings		0		0
Net unrealized investment gains (losses):
Included in earnings		0		0
Policyholders' account balances
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(41,946)	(31,978)	(46,651)	(20,337)
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	2,940	(17,026)	10,491	(16,531)
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	(60,847)	(36,525)	(60,847)	(36,525)
Policyholders' account balances | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(21,841)	(6,209)	(24,687)	(18,345)
Net unrealized investment gains (losses):
Included in earnings	(21,841)	(6,209)	(24,688)	(18,345)
Policyholders' account balances | 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
Policyholders' account balances | Interest credited to polcyholder account balances
Total gains (losses) (realized/unrealized):
Included in earnings		18,688		18,688
Net unrealized investment gains (losses):
Included in earnings		7,017		18,688
Other liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(17,610)	0	0	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	8,778	0	26,483	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	0	0	0	0
Fair Value, end of period	(41,333)	0	(41,333)	0
Other liabilities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(32,501)	0	(67,816)	0
Net unrealized investment gains (losses):
Included in earnings	(32,750)	0	(67,796)	0
Other liabilities | 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 liabilities | Interest credited to polcyholder account balances
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 | US government
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	24,411	11,552	19,204	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	935	3,358	6,142	11,340
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	0	0	3,570
Fair Value, end of period	25,346	14,910	25,346	14,910
Available-for-sale | Fixed maturities | US government | 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 | US government | 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
Available-for-sale | Fixed maturities | Foreign government bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	160		0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		(15)
Net investment income	0		1
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	(1)		173
Fair Value, end of period	159		159
Available-for-sale | Fixed maturities | 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
Available-for-sale | Fixed maturities | 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
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	62,418	45,375	75,421	102,556
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(2,249)	115	(2,839)	1,613
Net investment income	154	30	289	89
Purchases	669	1,841	4,816	13,666
Sales	0	0	(201)	(52,197)
Issuances	0	0	0	0
Settlements	(12,767)	(2,396)	(32,838)	(15,049)
Transfers into Level 3	0	3,223	4,028	5,204
Transfers out of Level 3	(3,936)	(1,099)	(4,145)	(4,691)
Other	0	(12)	14	(3,582)
Fair Value, end of period	43,467	46,939	43,467	46,939
Available-for-sale | Fixed maturities | Corporate securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(822)	(138)	(1,078)	(670)
Net unrealized investment gains (losses):
Included in earnings	(1,310)	(157)	(1,385)	(1,265)
Available-for-sale | Fixed maturities | Corporate 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
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	19,661	68,350	111,028	19,856
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(426)	187	(528)	(3,455)
Net investment income	(11)	9	17	97
Purchases	20,229	37,532	31,067	66,174
Sales	(10,648)	0	(10,844)	(7,471)
Issuances	0	0	0	0
Settlements	(2,138)	(3,317)	(10,204)	(46,117)
Transfers into Level 3	0	2,767	13,513	75,143
Transfers out of Level 3	0	(1,506)	(107,403)	(4,253)
Other	0	0	0	0
Fair Value, end of period	26,680	104,035	26,680	104,035
Available-for-sale | Fixed maturities | Structured securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	13	13	34	4,061
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | Structured 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