Employee Benefit Plans (Restricted Stock Unit Activity) (Details) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of RSUs (in thousands)
Beginning balance (in shares)	1,775,900	1,823,800	1,727,300
Granted (in shares)	467,400	642,500	640,400
Vested and issued (in shares)	(565,000.0)	(602,500)	(370,600)
Forfeited (in shares)	(85,700)	(87,900)	(173,300)
Beginning balance (in shares)	1,592,600	1,775,900	1,823,800
Number of Restricted Stock Units, Vested and unissued (in shares)	578,400
Weighted-Average Grant Date Fair Value per Share
Unissued, beginning balance (in dollars per share)	$ 45.48	$ 37.49	$ 34.15
Granted (in dollars per share)	64.32	59.87	45.01
Vested and Issued (in dollars per share)	41.50	36.26	30.03
Forfeited (in dollars per share)	54.84	47.86	38.77
Unissued, ending balance (in dollars per share)	51.88	$ 45.48	$ 37.49
Weighted Average Grant Date Fair Value per Share, Vested and unissued (in dollars per share)	$ 39.24
Weighted Average Remaining Contractual Term (years), Unissued	1 year
Aggregate Intrinsic Value, Unissued	$ 114.8