Employee Benefit Plans (Performance Shares Activity) (Details) - Performance Shares - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of PSUs (in thousands)
Beginning balance (in shares)	798,700	504,800	143,400
Granted (in shares)	374,900	316,100	401,200
Vested and issued (in shares)	(150,700)	0	0
Forfeited (in shares)	(23,200)	(22,200)	(39,800)
Beginning balance (in shares)	999,700	798,700	504,800
Number of PSUs, Vested and unissued (in shares)	17,100
Weighted-Average Grant Date Fair Value per Share
Beginning balance (in dollars per share)	$ 50.52	$ 44.40	$ 38.69
Granted (in dollars per share)	62.26	60.48	46.27
Vested and Issued (in dollars per share)	39.05	0	0
Forfeited (in dollars per share)	59.76	53.79	43.60
Ending balance (in dollars per share)	55.77	$ 50.52	$ 44.40
Weighted Average Grant Date Fair Value per Share, Vested and unissued (in dollars per share)	$ 38.69