Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 159,366	$ 111,511	$ 126,452
Other comprehensive income (loss), before tax:
Unrealized gains (losses) arising during the period	2,599	1,920	(20,231)
Amount reclassified from accumulated other comprehensive loss	180	88	(235)
Total Minimum pension liability	2,779	2,008	(20,466)
Income tax (expense) benefit related to items of comprehensive income		(724)	3,254
Other comprehensive income (loss), net of tax	2,779	1,284	(17,212)
Comprehensive income	$ 162,145	$ 112,795	$ 109,240