Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Cash Flows [Abstract]
Income tax refund received	$ 4,476	$ 815
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	363,703	522,319
Restricted cash	1,755	1,392
Cash, cash equivalents and restricted cash	$ 365,458	$ 523,711