Equity and Capital - Terms and Conditions of Preferred and Common Stock (Details) (USD $)	0 Months Ended	12 Months Ended																		0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended
	Feb. 09, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Series 3	Dec. 31, 2011 Series 3	Dec. 31, 2012 Series 4	Dec. 31, 2011 Series 4	Dec. 31, 2012 Series 5	Dec. 31, 2011 Series 5	Dec. 31, 2012 Series 6 and Series 7	Dec. 31, 2012 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member] Series 3	Dec. 31, 2011 Parent Company [Member] Series 4	Dec. 31, 2012 Parent Company [Member] Series 3 and Series 4	Dec. 31, 2012 Parent Company [Member] Series 5	Dec. 31, 2011 Parent Company [Member] Series 5	Feb. 16, 2012 Parent Company [Member] Series 6	Dec. 31, 2012 Parent Company [Member] Series 6	Aug. 23, 2012 Parent Company [Member] Series 7	Dec. 31, 2012 Parent Company [Member] Series 7	Dec. 31, 2012 Maximum [Member] Parent Company [Member]	Dec. 31, 2012 ATM Parent Company [Member]	Dec. 31, 2012 ATM Maximum [Member] Parent Company [Member]	Aug. 10, 2012 ATM Maximum [Member] Parent Company [Member]
Class of Stock [Line Items]
Shares issued												13,000,000	11,000,000		3,000,000	5,000,000			3,000,000	10,000,000	10,000,000	3,000,000	3,000,000		442,786
Distribution rate	3.75%		7.45%			7.45%		7.25%		6.70%										6.625%	6.625%	6.00%	6.00%
Preferred Stock, Callable Date		Sep. 29, 2009			Apr. 03, 2008		Aug. 31, 2009		Aug. 02, 2010												Feb. 16, 2017		Aug. 23, 2017
Preferred stock, liquidation preferences per share												$ 25	$ 25							$ 25		$ 25
Net proceeds from issuance of preferred stock		$ 313,900,000	$ 0	$ 0																$ 241,400,000		$ 72,500,000
Non-cash chages related to orginal issuance costs																	7,000,000	2,300,000
Amount avaiable for issuance																										128,000,000	150,000,000
Weighted average exercise price																									$ 49.70
Net proceeds from common stock issuance												21,542,000	215,369,000	0											21,500,000
Payments for commission expense																									331,000
Payments of stock issuance costs																									135,000
Liquidation preference		$ 325,000,000											$ 275,000,000		$ 75,000,000	$ 125,000,000			$ 75,000,000		$ 250,000,000		$ 75,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period											5 years													5 years