Borrowings - Senior Secured Revolving Credit Facility (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Senior secured revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 1,250,000,000
Remaining borrowing capacity	$ 1,200,000,000
Weighted average interest rate on facilities	4.60%	4.60%
Commitment fee percentage for the unused portion of the facility	0.50%
Senior secured revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	3.50%
Senior secured revolving credit facility | Base rate
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	2.50%
Letters of credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 250,000,000
Letters of credit outstanding amount	$ 41,000,000	$ 42,000,000