Subsequent Event (Details) (Preferred Stock, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended	0 Months Ended
	Mar. 31, 2013 Series A	Dec. 31, 2012 Series A	May 16, 2013 Subsequent event Series A Forecast	Apr. 15, 2013 Subsequent event Series B
Subsequent event
Preferred stock, issuance (in shares)				8,000,000
Preferred stock, dividend rate (as a percent)	8.50%	8.50%		7.50%
Preferred stock, liquidation preference (in dollars per share)	$ 25.00			$ 25.00
Estimated aggregate net proceeds from the offering of preferred stock				$ 193.3
Outstanding preferred stock that will be redeemed (in shares)			3,840,000
Preferred stock, redemption price (in dollars per share)	$ 25.00		$ 25.00
Preferred stock, aggregate redemption price (in dollars per share)			$ 25.27153
Aggregate redemption amount			$ 97.0