Accumulated Other Comprehensive Income (Loss) - Reclassification Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on investments										$ 319	$ 1,165	$ 658
Tax effect										(45,898)	(21,250)	(10,493)
Income from continuing operations		$ (21,814)	$ 44,758	$ 27,438	$ 7,593	$ (4,136)	$ 34,013	$ 19,873	$ (6,797)	58,452	43,479	$ 27,442
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										(100)	(97)
Tax effect										38	37
Income from continuing operations										(62)	(60)
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]									507	1,440
Amortization of prior service credit	[1]									(1,247)	(575)
Tax effect										310	(328)
Income from continuing operations										$ (430)	$ 537

[1] Amount included in pension expense. Refer to Note 17 – Pension and Postretirement Benefits