Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2018 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	$ 49,871,428	[1]
Amount at which shown in the balance sheet	49,427,498
Total Fixed Maturity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	46,131,190	[1]
Fair value	45,923,727
Amount at which shown in the balance sheet	45,923,727
United States Government Full Faith and Credit
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	11,872	[1]
Fair value	11,652
Amount at which shown in the balance sheet	11,652
United States Government Sponsored Agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	1,208,468	[1]
Fair value	1,138,529
Amount at which shown in the balance sheet	1,138,529
United States Municipalities, States and Territories
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	3,880,703	[1]
Fair value	4,126,267
Amount at which shown in the balance sheet	4,126,267
Foreign Government Obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	226,860	[1]
Fair value	230,274
Amount at which shown in the balance sheet	230,274
Corporate Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	28,483,138	[1]
Fair value	28,371,514
Amount at which shown in the balance sheet	28,371,514
Residential Mortgage Backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	1,134,623	[1]
Fair value	1,202,159
Amount at which shown in the balance sheet	1,202,159
Commercial Mortgage Backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	5,492,271	[1]
Fair value	5,379,003
Amount at which shown in the balance sheet	5,379,003
Other Asset Backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	5,693,255	[1]
Fair value	5,464,329
Amount at which shown in the balance sheet	5,464,329
Mortgage Loans on Real Estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	2,943,091	[1]
Fair value	2,920,612
Amount at which shown in the balance sheet	2,943,091
Derivative Instruments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	441,616	[1]
Fair value	205,149
Amount at which shown in the balance sheet	205,149
Other Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	355,531	[1]
Amount at which shown in the balance sheet	$ 355,531

[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.