Long-term Debt and Derivatives (Details 2) (USD $)	3 Months Ended				6 Months Ended					1 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010	Jul. 14, 2011 Revolving credit facility Issuance of debt	Jul. 14, 2011 Variable rate Term Loan A Issuance of debt	Jul. 14, 2011 Variable rate Term Loan B Issuance of debt	Jul. 31, 2011 Issuance of debt	Jul. 14, 2011 Issuance of debt
Future minimum repayments of long-term debt
Within one year					$ 3,052,000
1-3 years					165,000
3-5 years					250,193,000
Over 5 years					1,844,726,000
Total minimum payments					2,098,136,000	2,171,123,000
Term loan outstanding					1,518,100,000
Letters of credit outstanding					26,500,000
Available borrowing capacity					614,100,000
Quarterly principal payment of term loan	387,800,000	387,800,000	387,800,000	354,900,000
Long-term Debt
Variable rate term loan								700,000,000	750,000,000
Maximum borrowing capacity							700,000,000	700,000,000			2,150,000,000
Number of term loans										2
Increase (decrease) in amount outstanding under senior credit facility					$ (71,000,000)