Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	$ 11,700,494	$ 9,771,673
Fixed maturities trading	335,751	289,752
Equity securities	26,247	20,613
Short-term investments	123,310	37,568
Other invested assets	334,737	348,541
Reinsurance recoverables	592,751	572,102
Receivables from parent and affiliates	15,676	46,381
Separate account assets	33,071,354	31,210,346
TOTAL ASSETS	56,370,730	54,677,844
Future policy benefits	10,346,448	9,368,986
Policyholders’ account balances	5,465,749	5,353,596
Payables to parent and affiliates	197,220	30,846
Other Liabilities	406,009	811,016
Total Liabilities	50,549,780	47,936,276
Future policy benefits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net Liability	9,317,000	8,332,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Asset	608,000	625,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Liability	9,925,000	8,957,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	11,700,494	9,771,673
Fixed maturities trading	335,751	289,752
Equity securities	16,247	10,613
Short-term investments	115,560	29,818
Cash equivalents	1,331,528	3,692,359
Other invested assets	2	42,978
Reinsurance recoverables	283,991	239,911
Receivables from parent and affiliates	3,171	37,193
Subtotal excluding separate account assets	13,786,744	14,114,297
Separate account assets	33,071,354	31,210,346
TOTAL ASSETS	46,858,098	45,324,643
Future policy benefits	9,316,905	8,332,474
Policyholders’ account balances	80,579	42,350
Payables to parent and affiliates	152,202	0
Other Liabilities	13,425	6,629
Total Liabilities	9,563,111	8,381,453
Assets Netting	(4,467,947)	(4,804,816)
Liabilities Netting	(2,095,054)	(2,134,160)
Netting	2,373,000	2,671,000
Fair Value, Measurements, Recurring | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Netting	(4,467,947)	(4,804,816)
Fair Value, Measurements, Recurring | Payables to parent and affiliates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Netting	(2,094,591)	(2,133,496)
Fair Value, Measurements, Recurring | Other liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Netting	(463)	(664)
Fair Value, Measurements, Recurring | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	6,437,194	4,884,091
Fair Value, Measurements, Recurring | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	135,099	131,164
Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	211,474	199,636
Fair Value, Measurements, Recurring | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	1,619,726	1,473,973
Fair Value, Measurements, Recurring | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	1,064,225	1,062,953
Fair Value, Measurements, Recurring | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	301,248	291,086
Fair Value, Measurements, Recurring | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	916,309	812,232
Fair Value, Measurements, Recurring | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	512,969	505,244
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	416,733	361,880
Fair Value, Measurements, Recurring | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	85,517	49,414
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fixed maturities trading	0	0
Equity securities	5,324	4,896
Short-term investments	0	0
Cash equivalents	499,687	1,098,903
Other invested assets	463	4,380
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Subtotal excluding separate account assets	505,474	1,108,179
Separate account assets	0	0
TOTAL ASSETS	505,474	1,108,179
Future policy benefits	0	0
Policyholders’ account balances	0	0
Payables to parent and affiliates	0	0
Other Liabilities	13,888	7,293
Total Liabilities	13,888	7,293
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	11,556,657	9,668,753
Fixed maturities trading	335,751	289,752
Equity securities	5,307	12
Short-term investments	115,560	29,818
Cash equivalents	831,841	2,593,456
Other invested assets	4,467,486	4,843,414
Reinsurance recoverables	0	0
Receivables from parent and affiliates	3,171	37,193
Subtotal excluding separate account assets	17,315,773	17,462,398
Separate account assets	33,071,354	31,210,346
TOTAL ASSETS	50,387,127	48,672,744
Future policy benefits	0	0
Policyholders’ account balances	0	0
Payables to parent and affiliates	2,246,793	2,133,496
Other Liabilities	0	0
Total Liabilities	2,246,793	2,133,496
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	6,428,415	4,875,959
Fair Value, Measurements, Recurring | Level 2 | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	135,099	131,164
Fair Value, Measurements, Recurring | Level 2 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	211,474	199,636
Fair Value, Measurements, Recurring | Level 2 | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	1,619,726	1,473,973
Fair Value, Measurements, Recurring | Level 2 | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	1,010,861	1,008,632
Fair Value, Measurements, Recurring | Level 2 | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	301,031	291,086
Fair Value, Measurements, Recurring | Level 2 | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	885,090	781,101
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	493,289	495,908
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	386,155	361,880
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	85,517	49,414
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	143,837	102,920
Fixed maturities trading	0	0
Equity securities	5,616	5,705
Short-term investments	0	0
Cash equivalents	0	0
Other invested assets	0	0
Reinsurance recoverables	283,991	239,911
Receivables from parent and affiliates	0	0
Subtotal excluding separate account assets	433,444	348,536
Separate account assets	0	0
TOTAL ASSETS	433,444	348,536
Future policy benefits	9,316,905	8,332,474
Policyholders’ account balances	80,579	42,350
Payables to parent and affiliates	0	0
Other Liabilities	0	0
Total Liabilities	9,397,484	8,374,824
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	8,779	8,132
Fair Value, Measurements, Recurring | Level 3 | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	53,364	54,321
Fair Value, Measurements, Recurring | Level 3 | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	217	0
Fair Value, Measurements, Recurring | Level 3 | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	31,219	31,131
Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	19,680	9,336
Fair Value, Measurements, Recurring | Level 3 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	30,578	0
Fair Value, Measurements, Recurring | Level 3 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale	0	0
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value investment measured at NAV per share	$ 8,200	$ 8,000