CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, dividend rate	7.50%	7.50%
Common Stock
Adjustments related to tax withholding for share-based compensation	$ 6.0	$ 3.6	$ 2.3
Shares paid for tax withholding for share based compensation	744,588	487,559	309,206
Preferred Stock | Series B Preferred Stock
Preferred stock, dividend rate	7.50%	7.50%
Preferred stock, liquidation preference (in dollars per share)	$ 25.00	$ 25