Debt (Credit Facilities) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Debt
Outstanding Borrowings	$ 1,169,840,000	$ 1,169,437,000
Long-term Debt, Gross	1,175,000,000	1,175,000,000
Unamortized debt issuance costs on term loans	(5,160,000)	(5,563,000)
Revolving Credit Facility
Debt
Maximum borrowing capacity	$ 300,000,000
Additional maturity term	1 year
Outstanding Borrowings	$ 0	0
Interest Rate	2.19%
Remaining Borrowing Capacity	$ 300,000,000	300,000,000
2013 Five-Year Term Loan
Debt
Maximum borrowing capacity	$ 400,000,000.0
Maturity term	5 years
Outstanding Borrowings	$ 400,000,000	400,000,000
Interest Rate	3.10%
2012 Five-Year Term Loan
Debt
Maximum borrowing capacity	$ 400,000,000
Maturity term	5 years
Outstanding Borrowings	$ 400,000,000	400,000,000
Interest Rate	2.72%
2012 Seven-Year Term Loan
Debt
Maximum borrowing capacity	$ 225,000,000
Maturity term	7 years
Outstanding Borrowings	$ 225,000,000	225,000,000
Interest Rate	4.04%
2014 Seven-Year Term Loan
Debt
Maximum borrowing capacity	$ 150,000,000.0
Maturity term	7 years
Outstanding Borrowings	$ 150,000,000	$ 150,000,000
Interest Rate	3.43%