Credit Facility (Details) - USD ($)	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Apr. 14, 2014
Line of Credit Facility [Line Items]
Maximum capacity on credit facility			$ 705,000,000
Additional borrowing capacity under certain circumstances			1,000,000,000
Credit facility	$ 635,000,000	$ 635,000,000
Term Loan
Line of Credit Facility [Line Items]
Maximum capacity on credit facility			305,000,000
Revolving Loan
Line of Credit Facility [Line Items]
Maximum capacity on credit facility			$ 400,000,000
Credit Facility
Line of Credit Facility [Line Items]
Credit facility	$ 305,000,000	$ 330,000,000
Effective interest rate	2.10%	2.08%
Remaining borrowing capacity	$ 20,200,000
Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Unused capacity commitment fee percentage threshold	50.00%
Below Threshold | Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Unused capacity fee percentage	0.15%
Above Threshold | Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Unused capacity fee percentage	0.25%
Line of Credit Facility, Interest Rate, Option One | Credit Facility | Maximum | LIBOR
Line of Credit Facility [Line Items]
Spread on variable rate basis	2.25%
Line of Credit Facility, Interest Rate, Option One | Credit Facility | Minimum | LIBOR
Line of Credit Facility [Line Items]
Spread on variable rate basis	1.50%
Line of Credit Facility, Interest Rate, Option Two | Credit Facility | Maximum | Base Rate
Line of Credit Facility [Line Items]
Spread on variable rate basis	1.25%
Line of Credit Facility, Interest Rate, Option Two | Credit Facility | Minimum | Base Rate
Line of Credit Facility [Line Items]
Spread on variable rate basis	0.50%
Line of Credit Facility, Base Rate, Option Two | Credit Facility | Federal Funds Effective Rate
Line of Credit Facility [Line Items]
Spread on variable rate basis	0.50%
Line of Credit Facility, Base Rate, Option Three | Credit Facility | One-Month LIBOR
Line of Credit Facility [Line Items]
Spread on variable rate basis	1.00%