Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	$ 46,849,339	[1]
Amount at which shown in the balance sheet	50,300,705
Fixed Maturity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	43,193,800	[1]
Fair value	45,449,449
Amount at which shown in the balance sheet	45,450,030
Fixed Maturity Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	43,116,759	[1]
Fair value	45,372,989
Amount at which shown in the balance sheet	45,372,989
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	11,861	[1]
Fair value	11,876
Amount at which shown in the balance sheet	11,876
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	1,308,290	[1]
Fair value	1,305,017
Amount at which shown in the balance sheet	1,305,017
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	3,804,360	[1]
Fair value	4,166,812
Amount at which shown in the balance sheet	4,166,812
Foreign Government Obligations [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	228,214	[1]
Fair value	239,360
Amount at which shown in the balance sheet	239,360
Corporate Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	28,127,653	[1]
Fair value	29,878,971
Amount at which shown in the balance sheet	29,878,971
Corporate Securities [Member] | Held For Investment [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	77,041	[1]
Fair value	76,460
Amount at which shown in the balance sheet	77,041
Residential Mortgage Backed Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	1,028,484	[1]
Fair value	1,105,567
Amount at which shown in the balance sheet	1,105,567
Commercial Mortgage Backed Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	5,531,922	[1]
Fair value	5,544,850
Amount at which shown in the balance sheet	5,544,850
Other Asset Backed Securities [Member] | Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	3,075,975	[1]
Fair value	3,120,536
Amount at which shown in the balance sheet	3,120,536
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	2,665,531	[1]
Fair value	2,670,037
Amount at which shown in the balance sheet	2,665,531
Derivative Instruments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	373,244	[1]
Fair value	1,568,380
Amount at which shown in the balance sheet	1,568,380
Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	616,764	[1]
Amount at which shown in the balance sheet	$ 616,764

[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.