Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	$ 38,944,350	[1]
Amount at which shown in the balance sheet	39,570,332
Fixed Maturity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	35,900,332	[1]
Fair value	36,487,216
Amount at which shown in the balance sheet	36,498,461
Fixed Maturity Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	35,823,710	[1]
Fair value	36,421,839
Amount at which shown in the balance sheet	36,421,839
United States Government Full Faith and Credit [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	470,567	[1]
Fair value	471,256
Amount at which shown in the balance sheet	471,256
United States Government Sponsored Agencies [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	1,386,219	[1]
Fair value	1,398,611
Amount at which shown in the balance sheet	1,398,611
United States Municipalities, States and Territories [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	3,422,667	[1]
Fair value	3,755,367
Amount at which shown in the balance sheet	3,755,367
Foreign Government Obligations [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	210,953	[1]
Fair value	212,565
Amount at which shown in the balance sheet	212,565
Corporate Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	23,597,530	[1]
Fair value	23,802,394
Amount at which shown in the balance sheet	23,802,394
Corporate Securities [Member] | Held For Investment [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	76,622	[1]
Fair value	65,377
Amount at which shown in the balance sheet	76,622
Residential Mortgage Backed Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	1,366,985	[1]
Fair value	1,462,072
Amount at which shown in the balance sheet	1,462,072
Commercial Mortgage Backed Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	4,238,265	[1]
Fair value	4,174,396
Amount at which shown in the balance sheet	4,174,396
Other Asset Backed Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	1,130,524	[1]
Fair value	1,145,178
Amount at which shown in the balance sheet	1,145,178
Equity Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	7,515	[1]
Fair value	7,828
Amount at which shown in the balance sheet	7,828
Common Stocks [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	7,515	[1]
Fair value	7,828
Amount at which shown in the balance sheet	7,828
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	2,435,257	[1]
Fair value	2,471,864
Amount at which shown in the balance sheet	2,435,257
Derivative Instruments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	309,716	[1]
Fair value	337,256
Amount at which shown in the balance sheet	337,256
Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	291,530	[1]
Amount at which shown in the balance sheet	$ 291,530

[1] On the basis of cost adjusted for other than temporary impairments, repayments and amortization of premiums and accrual of discounts for fixed maturity securities and short-term investments, original cost for derivative instruments and unpaid principal balance less allowance for credit losses for mortgage loans.