Investments - Aging of Unrealized Losses (Details) $ in Thousands	Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months \| security	161	49
Fair value, less than 12 months	$ 359,709	$ 154,562
Unrealized losses, less than 12 months	$ (15,043)	$ (1,334)
Number of securities,12 months or greater \| security	6	16
Fair value, 12 months or greater	$ 6,228	$ 27,963
Unrealized losses, 12 months or greater	$ (94)	$ (84)
Number of securities, total \| security	167	65
Fair value	$ 365,937	$ 182,525
Unrealized Losses	$ (15,137)	$ (1,418)
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months \| security	0	4
Fair value, less than 12 months	$ 0	$ 12,001
Unrealized losses, less than 12 months	$ 0	$ (58)
Number of securities,12 months or greater \| security	0	0
Fair value, 12 months or greater	$ 0	$ 0
Unrealized losses, 12 months or greater	$ 0	$ 0
Number of securities, total \| security	0	4
Fair value	$ 0	$ 12,001
Unrealized Losses	$ 0	$ (58)
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months \| security	14	26
Fair value, less than 12 months	$ 46,936	$ 92,844
Unrealized losses, less than 12 months	$ (668)	$ (1,034)
Number of securities,12 months or greater \| security	0	1
Fair value, 12 months or greater	$ 0	$ 999
Unrealized losses, 12 months or greater	$ 0	$ (1)
Number of securities, total \| security	14	27
Fair value	$ 46,936	$ 93,843
Unrealized Losses	$ (668)	$ (1,035)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months \| security	87	10
Fair value, less than 12 months	$ 178,776	$ 30,481
Unrealized losses, less than 12 months	$ (6,630)	$ (140)
Number of securities,12 months or greater \| security	5	14
Fair value, 12 months or greater	$ 3,294	$ 23,976
Unrealized losses, 12 months or greater	$ (28)	$ (71)
Number of securities, total \| security	92	24
Fair value	$ 182,070	$ 54,457
Unrealized Losses	$ (6,658)	$ (211)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months \| security	60	9
Fair value, less than 12 months	$ 133,997	$ 19,236
Unrealized losses, less than 12 months	$ (7,745)	$ (102)
Number of securities,12 months or greater \| security	1	1
Fair value, 12 months or greater	$ 2,934	$ 2,988
Unrealized losses, 12 months or greater	$ (66)	$ (12)
Number of securities, total \| security	61	10
Fair value	$ 136,931	$ 22,224
Unrealized Losses	$ (7,811)	$ (114)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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