Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,403,404	$ 1,217,536	$ 1,129,449
Total other comprehensive income (loss)	16,257	8,346	30,387
Balance at end of period	1,690,779	1,403,404	1,217,536
Defined Benefit Pension
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	1,138	(1,425)
Other comprehensive income (loss) before reclassifications	5,075	2,468
Amounts reclassified from accumulated other comprehensive (loss) income	90	95
Total other comprehensive income (loss)	5,165	2,563
Balance at end of period	6,303	1,138	(1,425)
Foreign Currency Translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	5,124	(659)
Other comprehensive income (loss) before reclassifications	11,092	5,783
Amounts reclassified from accumulated other comprehensive (loss) income	0	0
Total other comprehensive income (loss)	11,092	5,783
Balance at end of period	16,216	5,124	(659)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	6,262	(2,084)
Other comprehensive income (loss) before reclassifications	16,167	8,251
Amounts reclassified from accumulated other comprehensive (loss) income	90	95
Total other comprehensive income (loss)	16,257	8,346
Balance at end of period	$ 22,519	$ 6,262	$ (2,084)