Long-term Debt - Senior Unsecured Credit Facility (Narrative) (Details) - USD ($)	Mar. 31, 2016	Dec. 31, 2015	Jul. 31, 2015
Long-term debt
Line of credit facility, maximum borrowing capacity	$ 1,000,000,000
Long-term debt	2,468,985,000	$ 2,510,341,000
Letters of credit outstanding, amount	900,000
Available borrowing capacity	551,100,000
Senior unsecured credit facility
Long-term debt
Long-term debt	448,000,000	$ 490,000,000
Revolving credit facility
Long-term debt
Line of credit facility, maximum borrowing capacity	700,000,000
Outstanding balance on credit facility	148,000,000
Term Loan a Facility
Long-term debt
Line of credit facility, maximum borrowing capacity	300,000,000
Outstanding balance on credit facility	$ 300,000,000
Term Loan A -1 Facility [Member]
Long-term debt
Line of credit facility, increase in borrowing capacity			$ 825,000,000