Investments - Schedule of Amortized Cost and Fair Value of Investments Available for Sale by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Amortized Cost	$ 4,616,898	$ 4,404,690
Fair Value
Fair Value	4,701,476	4,565,373
U.S. Treasury securities
Amortized Cost
Due in 1 year	51,984
Due after 1 but within 5 years	155,396
Due after 5 but within 10 years	49,036
Due after 10 years	985
Subtotal	257,401
Amortized Cost	257,401	259,378
Fair Value
Due in 1 year	52,264
Due after 1 but within 5 years	158,188
Due after 5 but within 10 years	50,890
Due after 10 years	967
Subtotal	262,309
U.S. agency securities
Amortized Cost
Due in 1 year	12,530
Due after 1 but within 5 years	3,498
Subtotal	16,028
Amortized Cost	16,028	17,930
Fair Value
Due in 1 year	12,638
Due after 1 but within 5 years	3,500
Subtotal	16,138
Municipal debt securities
Amortized Cost
Due in 1 year	7,495
Due after 1 but within 5 years	98,704
Due after 5 but within 10 years	229,338
Due after 10 years	208,205
Subtotal	543,742
Amortized Cost	543,742	513,870
Fair Value
Due in 1 year	7,519
Due after 1 but within 5 years	103,013
Due after 5 but within 10 years	243,858
Due after 10 years	217,873
Subtotal	572,263
Non-U.S. government securities:
Amortized Cost
Due in 1 year	4,360
Due after 1 but within 5 years	18,348
Due after 5 but within 10 years	26,603
Due after 10 years	28,129
Subtotal	77,440
Amortized Cost	77,440	56,045
Fair Value
Due in 1 year	4,473
Due after 1 but within 5 years	19,499
Due after 5 but within 10 years	29,045
Due after 10 years	26,263
Subtotal	79,280
Corporate debt securities
Amortized Cost
Due in 1 year	147,203
Due after 1 but within 5 years	648,697
Due after 5 but within 10 years	378,395
Due after 10 years	217,586
Subtotal	1,391,881
Amortized Cost	1,391,881	1,070,027
Fair Value
Due in 1 year	148,503
Due after 1 but within 5 years	667,465
Due after 5 but within 10 years	386,311
Due after 10 years	212,659
Subtotal	1,414,938
U.S. agency mortgage-backed securities
Amortized Cost
Without single maturity date	1,005,599
Amortized Cost	1,005,599	963,670
Fair Value
Without single maturity date	1,022,991
Residential and commercial mortgage securities
Amortized Cost
Without single maturity date	439,583
Amortized Cost	439,583	391,921
Fair Value
Without single maturity date	446,208
Asset-backed securities
Amortized Cost
Without single maturity date	471,677
Amortized Cost	471,677	452,527
Fair Value
Without single maturity date	473,804
Money market funds
Amortized Cost
Without single maturity date	413,547
Amortized Cost	413,547	$ 679,322
Fair Value
Without single maturity date	$ 413,545