Schedule I - Summary of Investments Other Than investments in Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2020		Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 16,177,891		$ 12,465,746
Fair Value	18,584,685		13,202,365
Equity securities, at cost	279,096		63,647
Equity securities, at fair value	288,082		67,503
Fixed Maturities, Trading, Amortized Cost	1,017,771		349,428
Fixed maturities, trading	1,114,142		383,198
Commercial mortgage and other loans	1,765,770	[1]	1,471,522
Policy loans	11,806		12,366
Short-term investments	318,161		335,358
Other invested assets	818,810		474,013
Total Investment at Cost	20,389,305
Total investment per Balance Sheet	22,901,456		15,946,325
Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	16,177,891		12,465,746
Fair Value	18,584,685		13,202,365
Equity securities
Schedule of Investments [Line Items]
Equity securities, at cost	279,096
Equity securities, at fair value	288,082
Available-for-sale | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	16,177,891
Fair Value	18,584,685
Trading | Fixed maturities
Schedule of Investments [Line Items]
Fixed Maturities, Trading, Amortized Cost	1,017,771
Fixed maturities, trading	1,114,142
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	7,283,403		6,667,347
Fair Value	8,902,432		7,119,824
Obligations of U.S. states and their political subdivisions | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	258,135		252,304
Fair Value	277,491		259,682
Foreign government bonds | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	153,009		203,386
Fair Value	177,383		222,884
Asset-backed securities | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	984,318		443,767
Fair Value	992,583		444,438
Residential mortgage-backed securities | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	69,909		73,814
Fair Value	75,724		77,761
Commercial mortgage-backed securities | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	728,522		557,584
Fair Value	785,942		$ 578,289
Public utilities | Available-for-sale | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	676,111
Fair Value	764,122
All other corporate bonds | Available-for-sale | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	5,998,033
Fair Value	6,579,340
Redeemable preferred stock | Available-for-sale | Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	26,451
Fair Value	29,668
Other common stock | Equity securities
Schedule of Investments [Line Items]
Equity securities, at cost	10,371
Equity securities, at fair value	10,161
Mutual funds | Equity securities
Schedule of Investments [Line Items]
Equity securities, at cost	265,303
Equity securities, at fair value	273,924
Perpetual preferred stocks | Equity securities
Schedule of Investments [Line Items]
Equity securities, at cost	3,422
Equity securities, at fair value	$ 3,997

[1] December 31, 2020 amounts include the impacts of the January 1, 2020 adoption of ASU 2016-13. See Note 2 for details.