Investments - Summary of Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,282,585	$ 2,793,350
Unrealized Gains	74,231	10,846
Unrealized Losses	(6,069)	(44,130)
Fair Value	3,350,747	2,760,066
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	239,087	295,145
Unrealized Gains	3,526	693
Unrealized Losses	(407)	(5,946)
Fair Value	242,206	289,892
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,620	33,645
Unrealized Gains	36	0
Unrealized Losses	(51)	(648)
Fair Value	33,605	32,997
U.S. agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	836,710	649,228
Unrealized Gains	13,956	2,520
Unrealized Losses	(2,332)	(14,570)
Fair Value	848,334	637,178
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	339,511	481,547
Unrealized Gains	22,245	5,351
Unrealized Losses	(118)	(3,019)
Fair Value	361,638	483,879
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	52,230	44,999
Unrealized Gains	2,812	285
Unrealized Losses	(47)	(283)
Fair Value	54,995	45,001
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	856,638	738,964
Unrealized Gains	24,255	1,005
Unrealized Losses	(592)	(14,768)
Fair Value	880,301	725,201
Residential and commercial mortgage securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	282,840	122,369
Unrealized Gains	6,542	686
Unrealized Losses	(1,101)	(1,217)
Fair Value	288,281	121,838
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	326,589	288,371
Unrealized Gains	857	305
Unrealized Losses	(1,421)	(3,679)
Fair Value	326,025	284,997
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	315,360	139,082
Unrealized Gains	2	1
Unrealized Losses	0	0
Fair Value	$ 315,362	$ 139,083

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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