Stockholders' Equity - Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on investments		$ 74	$ 44	$ 174	$ 138
Tax effect		(20,010)	(17,878)	(32,929)	(23,652)
Income from continuing operations		$ 45,842	$ 35,005	80,536	47,854
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains on Investments
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on investments				(72)	(67)
Tax effect				27	25
Income from continuing operations				(45)	(42)
Reclassification out of Accumulated Other Comprehensive Income | Unrecognized Net Actuarial Loss and Prior Service Credit, Net
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Recognized net actuarial (gain) loss	[1]			596	538
Amortization of prior service credit	[1]			(323)	(377)
Tax effect				(89)	(61)
Income from continuing operations				$ 184	$ 100

[1] Amount included in pension expense. Refer to Note 16 – Pension and Postretirement Benefits.