Real Estate Charges - Activity in the Liability Account Relating to Office Space Consolidation Initiatives (Details) $ in Thousands, ft² in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)
Restructuring and Related Activities [Abstract]
Square feet of office space sub-leased | ft²	1
Real estate liability restructuring reserve [Abstract]
Balance as of January 1,	$ 112,932	$ 123,912
Expense incurred	28,507	12,248
Deferred rent	7,083	4,930
Payments made	(39,122)	(32,988)
Interest accretion	4,235	4,830
Balance as of end of period	$ 113,635	$ 112,932