Investments - Fair Values and Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,059,143	$ 1,113,779
Gross Unrealized Gains	26,066	28,579
Gross Unrealized Losses	(15,137)	(1,418)
Fair Value	1,070,072	1,140,940
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	48,207	48,203
Gross Unrealized Gains	3,519	784
Gross Unrealized Losses	0	(58)
Fair Value	51,726	48,929
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	196,750	189,530
Gross Unrealized Gains	3,853	1,721
Gross Unrealized Losses	(668)	(1,035)
Fair Value	199,935	190,216
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	628,834	661,719
Gross Unrealized Gains	18,272	23,373
Gross Unrealized Losses	(6,658)	(211)
Fair Value	640,448	684,881
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	180,514	170,153
Gross Unrealized Gains	404	2,603
Gross Unrealized Losses	(7,811)	(114)
Fair Value	173,107	172,642
Total bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,054,305	1,069,605
Gross Unrealized Gains	26,048	28,481
Gross Unrealized Losses	(15,137)	(1,418)
Fair Value	1,065,216	1,096,668
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,838	44,174
Gross Unrealized Gains	18	98
Gross Unrealized Losses	0	0
Fair Value	$ 4,856	$ 44,272

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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Amortized cost of 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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