Equity - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014 class_of_stock	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Number of Classes of Common Stock													3
Common stock dividend, per share, cash paid	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.10	$ 0.1	$ 0.1	$ 0.10	$ 0.56	$ 0.48	$ 0.40
Common Class A, Voting [Member]
Class of Stock [Line Items]
Common Stock, Maximum Ownership Percentage by a Beneficial Owner	33.00%												33.00%
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, shares issued	2,400,000				2,400,000								2,400,000	2,400,000
Preferred Stock, shares outstanding	2,400,000				2,400,000								2,400,000	2,400,000
Preferred Stock, Dividend Rate, Percentage													5.875%
Payments of Stock Issuance Costs														$ 1.7
Preferred Stock, Par or Stated Value Per Share	$ 25				$ 25								$ 25	$ 25
Preferred Stock, Liquidation Preference Per Share	$ 25												$ 25
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, shares issued	3,000,000				0								3,000,000	0
Preferred Stock, shares outstanding	3,000,000				0								3,000,000	0
Preferred Stock, Dividend Rate, Percentage													6.875%
Payments of Stock Issuance Costs													1.9
Preferred Stock, Par or Stated Value Per Share	$ 25				$ 0								$ 25	$ 0
Preferred Stock, Liquidation Preference Per Share	$ 25												$ 25
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, shares issued	3,000,000				0								3,000,000	0
Preferred Stock, shares outstanding	3,000,000				0								3,000,000	0
Payments of Stock Issuance Costs													$ 1.6
Preferred Stock, Par or Stated Value Per Share	$ 25				$ 0								$ 25	$ 0
Preferred Stock, Liquidation Preference Per Share	$ 25												$ 25
Retired Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 1,000												$ 1,000
Up to but excluding July 17, 2024 [Member] | Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage													6.00%
July 17, 2024, thereafter [Member] | Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Variable Rate, Percentage													3.26%