Stock Compensation Plans (Summary of Activity for Time-Based and Performance-Based RSUs) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Time-Based RSUs
Shares
Outstanding at beginning of year (in shares)	1,218,714
Granted (in shares)	411,290
Vested (in shares)	(312,226)
Forfeited (in shares)	(151,198)
Outstanding at end of year (in shares)	1,166,580	1,218,714
Weighted- Average Grant Date Fair Value
Outstanding at end of year (in dollars per share)	$ 35.92
Granted (in dollars per share)	55.28	$ 31.86	$ 40.36
Vested (in dollars per share)	36.55
Forfeited (in dollars per share)	40.38
Outstanding at end of year (in dollars per share)	$ 42.00	$ 35.92
Performance-Based RSUs
Shares
Outstanding at beginning of year (in shares)	410,746
Granted (in shares)	131,597
Vested (in shares)	(118,044)
Forfeited (in shares)	0
Outstanding at end of year (in shares)	424,299	410,746
Weighted- Average Grant Date Fair Value
Outstanding at end of year (in dollars per share)	$ 35.27
Granted (in dollars per share)	56.59	$ 29.18	$ 41.15
Vested (in dollars per share)	36.85
Forfeited (in dollars per share)	0.00
Outstanding at end of year (in dollars per share)	$ 41.44	$ 35.27