Subsequent Events (Details) (Subsequent event, USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended
	Oct. 31, 2012
Subsequent events
Net proceeds from the issuance of debt	$ 790.7
2018 Notes
Subsequent events
Aggregate principal amount of notes issued	350
Interest rate (as a percent)	2.00%
Debt issuance price as a percentage of principal amount	99.91%
Effective yield (as a percent)	2.017%
2022 Notes
Subsequent events
Aggregate principal amount of notes issued	$ 450
Interest rate (as a percent)	3.25%
Debt issuance price as a percentage of principal amount	99.382%
Effective yield (as a percent)	3.323%
Common stock
Subsequent events
Common stock, dividends declared (in dollars per share)	$ 0.1514375
Class E preferred stockholders
Subsequent events
Preferred stock, dividends declared (in dollars per share)	$ 0.140625
Class F preferred stockholders
Subsequent events
Preferred stock, dividends declared (in dollars per share)	$ 0.138021