Employee Benefit Plans (Restricted Stock Unit Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of PSUs (in thousands)
Number of Restricted Stock Units, Unissued, beginning balance (in shares)	1,823,800	1,727,300	1,456,200
Granted (in shares)	642,500	640,400	722,500
Vested and issued (in shares)	(602,500)	(370,600)	(405,100)
Forfeited (in shares)	(87,900)	(173,300)	(46,300)
Number of Restricted Stock Units, Unissued, ending balance (in shares)	1,775,900	1,823,800	1,727,300
Number of Restricted Stock Units, Vested and unissued (in shares)	682,800
Weighted-Average Grant Date Fair Value per Share
Unissued, beginning balance (in dollars per share)	$ 37.49	$ 34.15	$ 24.07
Granted (in dollars per share)	59.87	45.01	39.12
Vested and Issued (in dollars per share)	36.26	30.03	30.56
Forfeited (in dollars per share)	47.86	38.77	36.44
Unissued, ending balance (in dollars per share)	45.48	$ 37.49	$ 34.15
Weighted Average Grant Date Fair Value per Share, Vested and Unissued (in dollars per share)	$ 32.90
Weighted Average Remaining Contractual Term (years), Unissued	1 year 1 month 6 days
Aggregate Intrinsic Value, Unissued	$ 120.1