Stock Compensation Plans (Summary of Activity for Time-Based and Performance-Based RSUs) (Details)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Time-Based RSUs
Shares
Outstanding, beginning of period (in shares) | shares	1,166,580
Granted (in shares) | shares	401,830
Vested (in shares) | shares	(324,051)
Forfeited (in shares) | shares	(58,396)
Outstanding, end of period (in shares) | shares	1,185,963
Weighted-Average Grant Date Fair Value
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 42.00
Granted (in dollars per share) | $ / shares	67.30
Vested (in dollars per share) | $ / shares	40.10
Forfeited (in dollars per share) | $ / shares	47.65
Outstanding, end of period (in dollars per share) | $ / shares	$ 50.81
Performance-Based RSUs
Shares
Outstanding, beginning of period (in shares) | shares	424,299
Granted (in shares) | shares	120,286
Vested (in shares) | shares	(133,295)
Forfeited (in shares) | shares	0
Outstanding, end of period (in shares) | shares	411,290
Weighted-Average Grant Date Fair Value
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 41.44
Granted (in dollars per share) | $ / shares	70.13
Vested (in dollars per share) | $ / shares	41.15
Forfeited (in dollars per share) | $ / shares	0.00
Outstanding, end of period (in dollars per share) | $ / shares	$ 49.93