Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands		Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 46,290,215		$ 36,980,933
Gross Unrealized Gains		924,018		208,233
Gross Unrealized Losses		1,488,950		2,162,592
Allowance for Credit Losses		38,744	$ 16,969	40,414	$ 25,452	$ 3,348	$ 2,008
Fair Value	[1]	45,686,539		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		1,171,068		1,199,628
Gross Unrealized Gains		24,458		8,357
Gross Unrealized Losses		95,847		108,744
Allowance for Credit Losses		0	0	0	0	0	0
Fair Value		1,099,679		1,099,241
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		480,059		570,253
Gross Unrealized Gains		1,051		1,156
Gross Unrealized Losses		30,649		30,343
Allowance for Credit Losses		0		0
Fair Value		450,461		541,066
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		438,696		362,154
Gross Unrealized Gains		7,788		646
Gross Unrealized Losses		40,138		52,466
Allowance for Credit Losses		0	0	0	0	0	0
Fair Value		406,346		310,334
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		19,010,171		14,134,828
Gross Unrealized Gains		307,435		60,917
Gross Unrealized Losses		755,060		957,316
Allowance for Credit Losses		0		1
Fair Value		18,562,546		13,238,428
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		6,767,307		6,030,898
Gross Unrealized Gains		101,904		35,828
Gross Unrealized Losses		180,856		301,451
Allowance for Credit Losses		17,017		11,178
Fair Value		6,671,338		5,754,097
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		5,034,385		3,804,503
Gross Unrealized Gains		100,705		21,136
Gross Unrealized Losses		84,419		126,767
Allowance for Credit Losses		405		21
Fair Value		5,050,266		3,698,851
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		6,825,055		5,838,939
Gross Unrealized Gains		321,469		43,334
Gross Unrealized Losses		257,148		511,426
Allowance for Credit Losses		17,822		29,214
Fair Value		6,871,554		5,341,633
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		4,706,213		3,728,073
Gross Unrealized Gains		35,237		31,431
Gross Unrealized Losses		4,942		8,841
Allowance for Credit Losses		3,500	0	0	0	0	1
Fair Value		4,733,008		3,750,663
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,304,368		944,652
Gross Unrealized Gains		17,357		4,567
Gross Unrealized Losses		34,342		53,444
Allowance for Credit Losses		0	0	0	0	0	0
Fair Value		1,287,383		895,775
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		552,893		367,005
Gross Unrealized Gains		6,614		861
Gross Unrealized Losses		5,549		11,794
Allowance for Credit Losses		0	$ 0	0	$ 0	$ 5	$ 7
Fair Value		$ 553,958		$ 356,072

[1] See Note 4 for details of balances associated with variable interest entities.