Long-term Debt and Derivatives (Details 2) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Future minimum repayments of long-term debt
Within one year	$ 42,600,000
1-3 years	137,695,000
3-5 years	891,728,000
Over 5 years	1,031,638,000
Total minimum payments	2,103,661,000	2,044,914,000
Senior secured credit facility
Future minimum repayments of long-term debt
Total minimum payments	1,776,500,000	1,715,750,000
Long-term Debt
Term loan amount outstanding	23,900,000
Revolving credit facility
Long-term Debt
Revolving credit facility amount outstanding	369,000,000
Available borrowing capacity	307,100,000
Variable rate Term Loan A
Long-term Debt
Term loan amount outstanding	665,000,000
Variable rate Term Loan B
Long-term Debt
Term loan amount outstanding	$ 742,500,000