Real Estate Charges (Details) $ in Thousands, ft² in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)
Restructuring and Related Activities [Abstract]
Square feet of office space sub-leased | ft²	1
Real estate liability restructuring reserve [Abstract]
Balance as of beginning of period	$ 113,635	$ 112,932
Expense incurred	(264)	28,507
Deferred rent	0	7,083
Payments made	(8,981)	(39,122)
Interest accretion	1,090	4,235
Balance as of end of period	$ 105,480	$ 113,635