Investments - Schedule of Amortized Cost and Fair Value of Investments Available for Sale by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Amortized Cost
Amortized Cost	$ 4,952,466	$ 4,897,607
Fair Value
Fair Value	4,620,340	4,962,887
U.S. Treasury securities
Amortized Cost
Due in 1 year	108,482
Due after 1 but within 5 years	336,849
Due after 5 but within 10 years	39,085
Due after 10 years	5,032
Subtotal	489,448
Amortized Cost	489,448	447,926
Fair Value
Due in 1 year	107,709
Due after 1 but within 5 years	321,947
Due after 5 but within 10 years	36,247
Due after 10 years	4,243
Subtotal	470,146
U.S. agency securities
Amortized Cost
Due in 1 year	2,000
Due after 1 but within 5 years	0
Subtotal	2,000
Amortized Cost	2,000	5,501
Fair Value
Due in 1 year	2,000
Due after 1 but within 5 years	0
Subtotal	2,000
Municipal debt securities
Amortized Cost
Due in 1 year	5,969
Due after 1 but within 5 years	135,891
Due after 5 but within 10 years	179,701
Due after 10 years	259,845
Subtotal	581,406
Amortized Cost	581,406	598,764
Fair Value
Due in 1 year	5,971
Due after 1 but within 5 years	135,078
Due after 5 but within 10 years	173,078
Due after 10 years	226,645
Subtotal	540,772
Non-U.S. government securities:
Amortized Cost
Due in 1 year	10,451
Due after 1 but within 5 years	27,720
Due after 5 but within 10 years	8,885
Due after 10 years	24,322
Subtotal	71,378
Amortized Cost	71,378	77,366
Fair Value
Due in 1 year	10,491
Due after 1 but within 5 years	26,989
Due after 5 but within 10 years	8,440
Due after 10 years	19,215
Subtotal	65,135
Corporate debt securities
Amortized Cost
Due in 1 year	250,985
Due after 1 but within 5 years	650,651
Due after 5 but within 10 years	425,659
Due after 10 years	136,023
Subtotal	1,463,318
Amortized Cost	1,463,318	1,428,645
Fair Value
Due in 1 year	249,089
Due after 1 but within 5 years	627,318
Due after 5 but within 10 years	370,729
Due after 10 years	106,974
Subtotal	1,354,110
U.S. agency mortgage-backed securities
Amortized Cost
Without single maturity date	867,198
Amortized Cost	867,198	1,005,611
Fair Value
Without single maturity date	783,438
Residential and commercial mortgage securities
Amortized Cost
Without single maturity date	591,239
Amortized Cost	591,239	541,638
Fair Value
Without single maturity date	545,999
Asset-backed securities
Amortized Cost
Without single maturity date	645,854
Amortized Cost	645,854	582,144
Fair Value
Without single maturity date	618,115
Money market funds
Amortized Cost
Without single maturity date	240,625
Amortized Cost	240,625	$ 210,012
Fair Value
Without single maturity date	$ 240,625