Long-Term Debt (Senior Unsecured Notes and Credit Agreement) (Details) (USD $)	12 Months Ended													0 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 5.5% Senior Notes Due 2020 [Member]	Dec. 31, 2011 5.5% Senior Notes Due 2020 [Member]	Dec. 31, 2012 7% Senior Notes Due 2014 [Member]	Dec. 31, 2011 7% Senior Notes Due 2014 [Member]	Dec. 31, 2012 6.75% Senior Notes Due 2018 [Member]	Dec. 31, 2011 6.75% Senior Notes Due 2018 [Member]	Dec. 31, 2012 Revolving credit facility due 2016 [Member]	Dec. 31, 2011 Revolving credit facility due 2016 [Member]	Dec. 31, 2012 Term loan facility due 2016 [Member]	Dec. 31, 2011 Term loan facility due 2016 [Member]	Feb. 01, 2012 Senior Notes [Member] 5.5% Senior Notes Due 2020 [Member]	Dec. 31, 2012 Senior Notes [Member] 5.5% Senior Notes Due 2020 [Member]	Dec. 31, 2012 Senior Notes [Member] 7% Senior Notes Due 2014 [Member]	Dec. 31, 2012 Senior Notes [Member] 6.75% Senior Notes Due 2018 [Member]	Dec. 31, 2012 Loans Payable [Member] Term loan facility due 2016 [Member]	Dec. 31, 2012 Line of Credit [Member] Revolving credit facility due 2016 [Member]
Debt Instrument [Line Items]
Senior Notes				$ 350,000,000	$ 0	$ 0	$ 14,700,000	$ 395,600,000	$ 395,000,000
Percentage interest on debt instrument															5.50%	7.00%	6.75%
Maximum percentage of principal that can be redeemed with net cash proceeds from common stock offering														35.00%			35.00%
Redemption payment as a percentage of principal cash from common stock offering														105.50%			106.75%
Debt instrument, maturity date															Feb. 01, 2020		Apr. 15, 2018	Dec. 07, 2016	Dec. 07, 2016
Debt instrument, redemption payment as a percentage of principal																100.00%
Early repayment of senior notes aggregate consideration including interest paid																15,200,000
Term loan facility												500,000,000	500,000,000
Maximum borrowing capacity under revolving credit facility										1,200,000,000
Revolving credit facility, amount outstanding										540,000,000	495,000,000
Revolving credit facilities letter of credit sublimit										200,000,000
Letters of credit, amount outstanding	56,500,000
Loss on debt extinguishment	0	2,200,000	19,600,000
Write-off of deferred debt issuance costs	0	400,000	3,500,000
Additional borrowing capacity under revolving credit facility										$ 603,500,000
Basis spread on variable interest rates																		1.75%	1.75%
Commitment fee on undrawn amounts																			0.30%
Impact on credit spread from increase in leverage ratio																		0.25%	0.25%
Leverage Ratio, minimum threshold, current credit spread	2.0
Leverage ratio, maximum threshold, current credit spread	3.25
Leverage ratio, minimum threshold, increase in credit spread	3.25