Accumulated Other Comprehensive Loss - Summary of the Tax Effects Allocated to Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Loss
Reclassification adjustment for loss (gain) included in earnings, Before-tax-amount			$ 104	$ 1,050	$ (10,757)
Reclassification of other-than-temporary impairment loss included in earnings, Before-tax-amount		$ 1,660	403	1,723	769
Reclassification into retained earnings for the adoption of ASU, Before-tax-amount	[1]			932
Other comprehensive loss, Before-tax-amount		416	(172)	1,970	(6,017)
Other comprehensive loss, Tax (expense) or benefit	[2]	(105)	(75)	(811)	2,319
Total other comprehensive income (loss), net of tax		311	(247)	1,159	(3,698)
Unrealized (loss) gain on investments
Accumulated Other Comprehensive Loss
Unrealized (loss) gain on investments, Net-of-tax amount				(1,295)
Reclassification adjustment for loss (gain) included in earnings, Before-tax-amount			104	1,050	(10,757)
Reclassification adjustment for loss (gain) included in earnings, Tax (expense) or benefit			(40)	(266)	4,143
Reclassification adjustment for loss (gain) included in earnings, Net-of-tax amount			64	784	(6,614)
Reclassification of other-than-temporary impairment loss included in earnings, Before-tax-amount		1,660	403	1,723	769
Reclassification adjustment for other-than-temporary impairment loss included in earnings, Tax (expense) or benefit		(421)	(296)	(436)	(296)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, Net-of-tax amount		1,239	107	1,287	473
Other comprehensive loss, Before-tax-amount		416	(172)	1,970	(6,017)
Other comprehensive loss, Tax (expense) or benefit		(105)	(75)	(811)	2,319
Total other comprehensive income (loss), net of tax		311	(247)	1,159	(3,698)
Unrealized (loss) gain on investments | ASU 2016-01
Accumulated Other Comprehensive Loss
Reclassification into retained earnings for the adoption of ASU, Before-tax-amount				932
Reclassification into retained earnings for the adoption of ASU, Tax (expense) or benefit				(236)
Reclassification into retained earnings for the adoption of ASU, Net-of-tax amount				696
Unrealized (loss) gain on investments | ASU 2018-02
Accumulated Other Comprehensive Loss
Reclassification into retained earnings for the adoption of ASU, Tax (expense) or benefit				(313)
Reclassification into retained earnings for the adoption of ASU, Net-of-tax amount				(313)
Unrestricted available-for-sale, Debt securities | Unrealized (loss) gain on investments
Accumulated Other Comprehensive Loss
Unrealized (loss) gain on investments, Before-tax amount		(1,244)	(683)	(1,726)	3,967
Unrealized (loss) gain on investments, Tax (expense) or benefit		316	263	438	(1,526)
Unrealized (loss) gain on investments, Net-of-tax amount		$ (928)	(420)	(1,288)	2,441
Restricted | Unrealized (loss) gain on investments
Accumulated Other Comprehensive Loss
Unrealized (loss) gain on investments, Before-tax amount			4	(9)	4
Unrealized (loss) gain on investments, Tax (expense) or benefit			(2)	2	(2)
Unrealized (loss) gain on investments, Net-of-tax amount			$ 2	$ (7)	$ 2

[1] The reclassification into retained earnings relates to the adoption of Accounting Standards Update (“ASU”) 201601 Financial Instruments - Overall, as amended (“ASU 201601”). The new guidance was effective January 1, 2018, and required equity investments to be measured at fair value with changes in fair value recognized in results of operations rather than the condensed consolidated statements of comprehensive income. See Note 2. Summary of Significant Accounting Policies.

[2] Income tax expense for the nine months ended September 30, 2018 includes $0.3 million of income tax expense related to the adoption of ASU 201802 Income Statement - Reporting Comprehensive Income (“ASU 201802”). The new guidance was effective January 1, 2018, and allowed a reclassification from accumulated other comprehensive income (loss) to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act (the “Tax Act”). See Note 2. Summary of Significant Accounting Policies.