Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	6 Months Ended		3 Months Ended		6 Months Ended					1 Months Ended	3 Months Ended		6 Months Ended					6 Months Ended					1 Months Ended	6 Months Ended				1 Months Ended
	Feb. 29, 2012	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Series D Depositary Share	Jun. 30, 2013 Series D Depositary Share	Jun. 30, 2014 Series D Depositary Share		Jun. 30, 2013 Series D Depositary Share		Dec. 31, 2013 Series D Depositary Share	May 31, 2013 Series E Depositary Share	Jun. 30, 2014 Series E Depositary Share	Jun. 30, 2013 Series E Depositary Share	Jun. 30, 2014 Series E Depositary Share		Jun. 30, 2013 Series E Depositary Share		Dec. 31, 2013 Series E Depositary Share	Jun. 30, 2014 DRIP and ATM Equity Program	Jun. 30, 2013 DRIP and ATM Equity Program	Jun. 30, 2014 ATM Equity Program 2012	Jun. 30, 2013 ATM Equity Program 2012		May 31, 2012 ATM Equity Program 2012 Common Stock	Jun. 30, 2014 ATM Equity Program 2013		Jun. 30, 2013 ATM Equity Program 2013		Mar. 31, 2013 ATM Equity Program 2013 Common Stock	Jul. 31, 2014 Subsequent Event
Class of Stock [Line Items]
Dividends				$ 4,762	$ 4,762	$ 9,523	[1]	$ 9,523	[1]			$ 4,096	$ 0	$ 8,194	[1]	$ 0	[1]
Per share						$ 0.828125	[1]	$ 0.828125	[1]					$ 0.71250	[1]	$ 0.000000	[1]
Dividends		98,958	90,943
Per share		$ 0.810	$ 0.790
Shares of common stock																					0	4,676,452			1,861,206		2,280,450
Average price per share (net)																						$ 32.60			$ 35.21		$ 37.83
Net proceeds																						152,435			65,526		86,271
Stock issuance costs		1,104	13,416								9,856											2,161	[2]		1,065	[2]	1,551	[2]
Common stock, dividends, per share, declared (in dollars per share)																														$ 0.42
Preferred stock, shares issued (in shares)				11,500,000		11,500,000				11,500,000	11,500,000	11,500,000		11,500,000				11,500,000
Preferred stock, dividend rate, percentage											5.70%
Preferred stock, redemption price per share (in dollars per share)											$ 25.00
Proceeds from issuance of Series E preferred stock		0	287,500								287,500
Shares available for issuance under at market offering program (in shares)																								9,000,000					9,000,000
Common stock, shares reserved for future issuance, dividend reinvestment plan (in shares)	16,000,000
Shares of common stock																			290,399	475,866
Net proceeds		$ 76,098	$ 258,085																$ 9,814	$ 15,881

[1] The Series D and E preferred stock have no maturity date and will remain outstanding unless redeemed.

[2] Stock issuance costs consist primarily of underwriters' fees and commissions, and legal and accounting fees.