Stockholders' Equity - Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on investments		$ 42	$ 38	$ 100	$ 94
Tax effect		(10,178)	(9,226)	(12,919)	(5,774)
Income from continuing operations		$ 27,365	$ 19,808	34,694	12,849
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				(47)	(25)
Tax effect				18	9
Income from continuing operations				(29)	(16)
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]			439	388
Amortization of prior service credit	[1]			(216)	(251)
Tax effect				(75)	(52)
Income from continuing operations				$ 148	$ 85

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