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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator
Net income	$ 67.0	$ 120.2	$ 173.0	$ 264.0
Less: Preferred stock dividends	(4.8)	0.0	(4.8)	0.0
Net Income (Loss) Available to Common Stockholders, Basic	62.2	120.2	168.2	264.0
Less: Common stock dividends paid	(30.9)	(30.3)	(60.6)	(60.0)
Undistributed earnings	$ 31.3	$ 89.9	$ 107.6	$ 204.0
Denominator
Weighted average shares outstanding used in basic earnings per share (in shares)	57,060,313	55,230,367	55,125,584	55,713,172
Incremental common shares from:
Weighted average shares used in diluted earnings per share calculations (in shares)	57,264,408	55,509,898	57,273,428	56,075,152
Earnings per common share - Basic
Distributed earnings (in dollars per share)	$ 0.54	$ 0.55	$ 1.10	$ 1.08
Undistributed earnings (in dollars per share)	0.55	1.63	1.95	3.66
Net income (in dollars per share)	1.09	2.18	3.05	4.74
Earnings per common share - Diluted
Distributed earnings (in dollars per share)	0.54	0.54	1.06	1.07
Undistributed earning (in dollars per share)	0.55	1.62	1.96	3.64
Net income (in dollars per share)	$ 1.09	$ 2.16	$ 3.02	$ 4.71
PSUs
Incremental common shares from:
Incremental common shares (in shares)	163,596	239,400	217,455	321,849
ESPP
Incremental common shares from:
Incremental common shares (in shares)	40,499	40,131	40,499	40,131
Series D MCPS
Incremental common shares from:
Mandatory convertible preferred stock (in shares)	0	0	1,889,890	0