Equity - Preferred Stock (Details) - USD ($)	3 Months Ended			12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Excess Tax Benefits from Share-Based Compensation				$ 860,000	$ 0	$ 0
Dividends, Preferred Stock, Cash				$ 13,182,000	$ 13,182,000	$ 13,182,000
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, Dividend Rate, Percentage				5.875%
Payments of Stock Issuance Costs				$ 1,700,000
Preferred Stock, Shares Outstanding	2,400,000	2,400,000		2,400,000	2,400,000
Preferred stock, par value	$ 25	$ 25		$ 25	$ 25
Preferred Stock, Liquidation Preference Per Share	$ 25			$ 25
Dividends, Preferred Stock, Cash	$ 0.3672
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	3,000,000	3,000,000		3,000,000	3,000,000
Preferred Stock, Dividend Rate, Percentage				6.875%
Payments of Stock Issuance Costs				$ 1,900,000
Preferred Stock, Shares Outstanding	3,000,000	3,000,000		3,000,000	3,000,000
Preferred stock, par value	$ 25	$ 25		$ 25	$ 25
Dividends, Preferred Stock, Cash		$ 0.4297
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	3,000,000	3,000,000		3,000,000	3,000,000
Payments of Stock Issuance Costs				$ 1,600,000
Preferred Stock, Shares Outstanding	3,000,000	3,000,000		3,000,000	3,000,000
Preferred stock, par value	$ 25	$ 25		$ 25	$ 25
Dividends, Preferred Stock, Cash			$ 0.3750
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%