Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 43,550,555		$ 36,980,933
Gross Unrealized Gains	729,737		208,233
Gross Unrealized Losses	1,710,917		2,162,592
Allowance for Credit Losses	16,969	$ 42,828	40,414	$ 3,348	$ 1,650	$ 2,008
Fair Value	42,552,406		34,986,160
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,150,774		1,199,628
Gross Unrealized Gains	21,752		8,357
Gross Unrealized Losses	102,875		108,744
Allowance for Credit Losses	0	0	0	0	0	0
Fair Value	1,069,651		1,099,241
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	485,505		570,253
Gross Unrealized Gains	507		1,156
Gross Unrealized Losses	44,965		30,343
Allowance for Credit Losses	0		0
Fair Value	441,047		541,066
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	436,992		362,154
Gross Unrealized Gains	5,121		646
Gross Unrealized Losses	49,319		52,466
Allowance for Credit Losses	0	0	0	0	0	0
Fair Value	392,794		310,334
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,836,358		14,134,828
Gross Unrealized Gains	208,508		60,917
Gross Unrealized Losses	877,821		957,316
Allowance for Credit Losses	0		1
Fair Value	17,167,045		13,238,428
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,508,327		6,030,898
Gross Unrealized Gains	76,363		35,828
Gross Unrealized Losses	204,863		301,451
Allowance for Credit Losses	10,493		11,178
Fair Value	6,369,334		5,754,097
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,542,812		3,804,503
Gross Unrealized Gains	74,358		21,136
Gross Unrealized Losses	104,900		126,767
Allowance for Credit Losses	21		21
Fair Value	4,512,249		3,698,851
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,494,932		5,838,939
Gross Unrealized Gains	292,364		43,334
Gross Unrealized Losses	268,467		511,426
Allowance for Credit Losses	6,455		29,214
Fair Value	6,512,374		5,341,633
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,380,196		3,728,073
Gross Unrealized Gains	33,007		31,431
Gross Unrealized Losses	8,777		8,841
Allowance for Credit Losses	0	0	0	0	0	1
Fair Value	4,404,426		3,750,663
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,168,631		944,652
Gross Unrealized Gains	13,927		4,567
Gross Unrealized Losses	41,387		53,444
Allowance for Credit Losses	0	0	0	0	0	0
Fair Value	1,141,171		895,775
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	546,028		367,005
Gross Unrealized Gains	3,830		861
Gross Unrealized Losses	7,543		11,794
Allowance for Credit Losses	0	$ 0	0	$ 5	$ 0	$ 7
Fair Value	$ 542,315		$ 356,072