Recent Developments (Details) - USD ($)					12 Months Ended
	Feb. 05, 2019	Sep. 19, 2018	Sep. 15, 2016	Jun. 15, 2015	Dec. 31, 2018	Sep. 18, 2018	Apr. 24, 2018	Jul. 25, 2017	Aug. 28, 2015
Series C Preferred Stock
Recent Developments [Line Items]
Preferred Stock, Dividend Rate, Percentage				6.125%	6.125%
Stock repurchase program authorized amount									$ 10,000,000
Remaining authorized repurchase amount					$ 10,000,000
Series D Preferred Stock
Recent Developments [Line Items]
Preferred Stock, Dividend Rate, Percentage			5.375%		5.375%
Stock repurchase program authorized amount								$ 15,000,000
Remaining authorized repurchase amount					$ 14,000,000
Series E Preferred Stock
Recent Developments [Line Items]
Preferred Stock, Dividend Rate, Percentage		5.875%			5.875%
Common Stock
Recent Developments [Line Items]
Stock repurchase program authorized amount						$ 200,000,000	$ 100,000,000
Remaining authorized repurchase amount					$ 111,000,000
Subsequent Event
Recent Developments [Line Items]
Common stock, dividends declared ($ per share)	$ 0.17
Subsequent Event | Series C Preferred Stock
Recent Developments [Line Items]
Preferred stock, dividends declared ($ per share)	0.3828125
Subsequent Event | Series D Preferred Stock
Recent Developments [Line Items]
Preferred stock, dividends declared ($ per share)	0.3359375
Subsequent Event | Series E Preferred Stock
Recent Developments [Line Items]
Preferred stock, dividends declared ($ per share)	$ 0.3671875
Subsequent Event | Common Stock
Recent Developments [Line Items]
Stock repurchase program authorized amount	$ 100,000,000
Remaining authorized repurchase amount	$ 211,000,000