Earnings Per Common Share - Net Income, Weighted Average Common Shares Used in Calculating Basic Earnings Per Common Share and Diluted EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator
Net income attributable to stockholders	$ 144.1	$ 67.0	$ 309.8	$ 173.0
Less: Preferred stock dividends	(4.6)	(4.8)	(9.3)	(4.8)
Net income attributable to common stockholders	139.5	62.2	300.5	168.2
Less: Common stock dividends paid	(38.3)	(30.9)	(75.7)	(60.6)
Undistributed earnings	$ 101.2	$ 31.3	$ 224.8	$ 107.6
Denominator
Weighted average common shares outstanding used in basic earnings per common share (in shares)	62,222,090	57,060,313	62,407,429	55,125,584
Incremental common shares from:
Weighted average common shares used in diluted earnings per common share calculations (in shares)	65,288,447	57,264,408	65,560,930	57,273,428
Earnings per common share - Basic
Distributed earnings (in dollars per share)	$ 0.62	$ 0.54	$ 1.21	$ 1.10
Undistributed earnings (in dollars per share)	1.62	0.55	3.61	1.95
Net income (in dollars per share)	2.24	1.09	4.82	3.05
Earnings per common share - Diluted
Distributed earnings (in dollars per share)	0.59	0.54	1.16	1.06
Undistributed earning (in dollars per share)	1.62	0.55	3.57	1.96
Net income (in dollars per share)	$ 2.21	$ 1.09	$ 4.73	$ 3.02
PSUs
Incremental common shares from:
Incremental common shares (in shares)	114,502	163,596	172,321	217,455
ESPP
Incremental common shares from:
Incremental common shares (in shares)	42,642	40,499	42,642	40,499
Series D MCPS
Incremental common shares from:
Mandatory convertible preferred stock (in shares)	2,909,213	0	2,938,538	1,889,890