Accumulated Other Comprehensive Loss - Summary of the Tax Effects Allocated to Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Loss
Reclassification adjustment for loss (gain) included in earnings, Before-tax-amount		$ (28)	$ (7,739)	$ 1,050	$ (10,861)
Reclassification of other-than-temporary impairment loss included in earnings, Before-tax-amount				63	366
Reclassification into retained earnings for the adoption of ASU, Before-tax-amount	[1]			932
Other comprehensive loss, Before-tax-amount		293	(6,998)	1,554	(5,845)
Other comprehensive loss, Tax (expense) or benefit	[2]	(74)	2,835	(706)	2,394
Total other comprehensive income (loss), net of tax		219	(4,163)	848	(3,451)
Unrealized gain (loss) on investments
Accumulated Other Comprehensive Loss
Unrealized gain (loss) on investments, Net-of-tax amount				(366)
Reclassification adjustment for loss (gain) included in earnings, Before-tax-amount		(28)	(7,739)	1,050	(10,861)
Reclassification adjustment for loss (gain) included in earnings, Tax (expense) or benefit		7	3,120	(266)	4,183
Reclassification adjustment for loss (gain) included in earnings, Net-of-tax amount		(21)	(4,619)	784	(6,678)
Reclassification of other-than-temporary impairment loss included in earnings, Before-tax-amount				63	366
Reclassification adjustment for other-than-temporary impairment loss included in earnings, Tax (expense) or benefit				(16)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, Net-of-tax amount				47	366
Other comprehensive loss, Before-tax-amount		293	(6,998)	1,554	(5,845)
Other comprehensive loss, Tax (expense) or benefit		(74)	2,835	(706)	2,394
Total other comprehensive income (loss), net of tax		219	(4,163)	848	(3,451)
Unrealized gain (loss) 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 gain (loss) 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 gain (loss) on investments
Accumulated Other Comprehensive Loss
Unrealized gain (loss) on investments, Before-tax amount		321	745	(482)	4,650
Unrealized gain (loss) on investments, Tax (expense) or benefit		(82)	(287)	123	(1,789)
Unrealized gain (loss) on investments, Net-of-tax amount		$ 239	458	(359)	$ 2,861
Restricted | Unrealized gain (loss) on investments
Accumulated Other Comprehensive Loss
Unrealized gain (loss) on investments, Before-tax amount			(4)	(9)
Unrealized gain (loss) on investments, Tax (expense) or benefit			2	2
Unrealized gain (loss) on investments, Net-of-tax amount			$ (2)	$ (7)

[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 requires 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 six months ended June 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 allows 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.