Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	$ 13,202,365	$ 9,771,673
Fixed maturities, trading	383,198	289,752
Equity securities	67,503	20,613
Other invested assets	474,013	348,541
Other assets	139,933	85,310
Reinsurance recoverables	621,510	572,102
Receivables from parent and affiliates	62,765	46,381
Separate account assets	32,665,431	31,210,346
TOTAL ASSETS	58,834,997	54,677,844
Future policy benefits	12,932,461	9,368,986
Policyholders’ account balances	6,180,359	5,353,596
Payables to parent and affiliates	185,156	30,846
Other liabilities	447,405	811,016
TOTAL LIABILITIES	53,307,642	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	11,823,000	8,332,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Asset	583,000	625,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Liability	12,406,000	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:	13,202,365	9,771,673
Fixed maturities, trading	383,198	289,752
Equity securities	57,503	10,613
Short-term investments	260,354	29,818
Cash equivalents	1,805,605	3,692,359
Other invested assets	2	42,978
Other assets	8,059	0
Reinsurance recoverables	349,820	239,911
Receivables from parent and affiliates	2,573	37,193
Subtotal excluding separate account assets	16,069,479	14,114,297
Separate account assets	32,665,431	31,210,346
TOTAL ASSETS	48,734,910	45,324,643
Future policy benefits	11,822,998	8,332,474
Policyholders’ account balances	196,892	42,350
Payables to parent and affiliates	145,947	0
Other liabilities	8,130	6,629
TOTAL LIABILITIES	12,173,967	8,381,453
Asset Netting	(7,324,049)	(4,804,816)
Liability Netting	(3,053,132)	(2,134,160)
Netting	4,271,000	2,671,000
Fair Value, Measurements, Recurring | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Netting	(7,324,049)	(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]
Liability Netting	(3,052,493)	(2,133,496)
Fair Value, Measurements, Recurring | Other liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability Netting	(639)	(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:	7,119,824	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:	259,682	131,164
Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	222,884	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:	1,740,676	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:	1,207,339	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:	337,987	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:	1,213,485	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:	444,438	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:	578,289	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:	77,761	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	5,314	4,896
Short-term investments	0	0
Cash equivalents	150,631	1,098,903
Other invested assets	639	4,380
Other assets	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Subtotal excluding separate account assets	156,584	1,108,179
Separate account assets	0	0
TOTAL ASSETS	156,584	1,108,179
Future policy benefits	0	0
Policyholders’ account balances	0	0
Payables to parent and affiliates	0	0
Other liabilities	8,509	7,293
TOTAL LIABILITIES	8,509	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 bonds
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:	13,068,726	9,668,753
Fixed maturities, trading	378,734	289,752
Equity securities	46,942	12
Short-term investments	260,354	29,818
Cash equivalents	1,654,974	2,593,456
Other invested assets	7,323,412	4,843,414
Other assets	0	0
Reinsurance recoverables	47,006	0
Receivables from parent and affiliates	2,573	37,193
Subtotal excluding separate account assets	22,782,721	17,462,398
Separate account assets	32,665,431	31,210,346
TOTAL ASSETS	55,448,152	48,672,744
Future policy benefits	0	0
Policyholders’ account balances	0	0
Payables to parent and affiliates	3,198,440	2,133,496
Other liabilities	260	0
TOTAL LIABILITIES	3,198,700	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:	7,109,277	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:	259,682	131,164
Fair Value, Measurements, Recurring | Level 2 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	222,884	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:	1,732,632	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:	1,155,464	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:	337,800	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:	1,169,324	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:	425,613	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:	578,289	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:	77,761	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:	133,639	102,920
Fixed maturities, trading	4,464	0
Equity securities	5,247	5,705
Short-term investments	0	0
Cash equivalents	0	0
Other invested assets	0	0
Other assets	8,059	0
Reinsurance recoverables	302,814	239,911
Receivables from parent and affiliates	0	0
Subtotal excluding separate account assets	454,223	348,536
Separate account assets	0	0
TOTAL ASSETS	454,223	348,536
Future policy benefits	11,822,998	8,332,474
Policyholders’ account balances	196,892	42,350
Payables to parent and affiliates	0	0
Other liabilities	0	0
TOTAL LIABILITIES	12,019,890	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:	10,547	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 bonds
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:	8,044	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:	51,875	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:	187	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:	44,161	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:	18,825	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:	0	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	$ 10,500	$ 8,000