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, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	$ 9,362,763	$ 2,524,272
Trading account assets	149,871	5,653
Equity securities, available-for-sale	18	17
Short-term investments	947,150	158,227
Other long-term investments	551,931	182,157
Receivables from parent and affiliates	111,703	212,696
Separate account assets	37,429,739	39,250,159
Future policy benefits	8,686,196	3,578,662
Netting	2,928,000	(3,800)
Embedded Derivative, Fair Value of Embedded Derivative, Net Liability	7,707,000	3,134,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Asset	1,060,000	34,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Liability	8,767,000	3,168,000
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, equity	10,358	5,653
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	9,362,763	2,524,272
Trading account assets	149,871	5,653
Equity securities, available-for-sale	18	17
Short-term investments	912,150	158,227
Cash equivalents	1,586,153	225
Other long-term investments	313,819	115,266
Reinsurance recoverables	240,091	3,012,653
Receivables from parent and affiliates	40,924	37,340
Subtotal excluding separate account assets	12,605,789	5,853,653
Separate account assets	37,429,739	39,250,159
Total assets	50,035,528	45,103,812
Future policy benefits	7,707,333	3,134,077
Payables to parent and affiliates	0	0
Other liabilities	5,051
Total liabilities	7,712,384	3,134,077
Asset Netting	(4,582,540)	(21,508)
Liability Netting	(1,654,360)	(25,277)
Fair Value, Measurements, Recurring | Other long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Netting	(4,582,540)	(21,508)
Fair Value, Measurements, Recurring | Payables to parent and affiliates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability Netting	(1,654,360)	(25,277)
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:	4,465,025	12,154
Trading account assets, debt	116,184	0
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:	89,974	20,212
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:	69,386	49,283
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:	1,925,038	949,109
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:	1,121,868	631,075
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:	217,363	136,222
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:	538,031	225,349
Fair Value, Measurements, Recurring | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	21,632	0
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:	251,309	151,290
Trading account assets, debt	1,697	0
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:	484,713	215,740
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:	200,056	133,838
Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, equity	10,358	5,653
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
Trading account assets	5,494	5,653
Equity securities, available-for-sale	0	0
Short-term investments	519,000	157,257
Cash equivalents	738,449	0
Other long-term investments	23,967	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Subtotal excluding separate account assets	1,286,910	162,910
Separate account assets	0	0
Total assets	1,286,910	162,910
Future policy benefits	0	0
Payables to parent and affiliates	0	0
Other liabilities	5,051
Total liabilities	5,051	0
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
Trading account assets, debt	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 | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	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
Trading account assets, debt	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 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, equity	5,494	5,653
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	9,178,952	2,350,471
Trading account assets	139,513	0
Equity securities, available-for-sale	18	17
Short-term investments	392,700	520
Cash equivalents	847,329	0
Other long-term investments	4,872,392	135,209
Reinsurance recoverables	0	0
Receivables from parent and affiliates	6,962	29,676
Subtotal excluding separate account assets	15,437,866	2,515,893
Separate account assets	37,429,739	39,250,159
Total assets	52,867,605	41,766,052
Future policy benefits	0	0
Payables to parent and affiliates	1,654,360	25,277
Other liabilities	0
Total liabilities	1,654,360	25,277
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:	4,465,025	12,154
Trading account assets, debt	116,184	0
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:	89,974	20,212
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:	69,299	49,283
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:	1,909,440	934,109
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:	997,004	523,298
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:	217,363	136,222
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:	526,504	220,818
Fair Value, Measurements, Recurring | Level 2 | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	21,632	0
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:	219,574	104,797
Trading account assets, debt	1,697	0
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:	484,713	215,740
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:	200,056	133,838
Fair Value, Measurements, Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, equity	0	0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale:	183,811	173,801
Trading account assets	4,864	0
Equity securities, available-for-sale	0	0
Short-term investments	450	450
Cash equivalents	375	225
Other long-term investments	0	1,565
Reinsurance recoverables	240,091	3,012,653
Receivables from parent and affiliates	33,962	7,664
Subtotal excluding separate account assets	463,553	3,196,358
Separate account assets	0	0
Total assets	463,553	3,196,358
Future policy benefits	7,707,333	3,134,077
Payables to parent and affiliates	0	0
Other liabilities	0
Total liabilities	7,707,333	3,134,077
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:	0	0
Trading account assets, debt	0	0
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:	87	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:	15,598	15,000
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:	124,864	107,777
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:	0	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:	11,527	4,531
Fair Value, Measurements, Recurring | Level 3 | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	0	0
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:	31,735	46,493
Trading account assets, debt	0	0
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
Fair Value, Measurements, Recurring | Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, equity	$ 4,864	$ 0