Stock-based Compensation (Summary of Restricted Stock Activity) (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, beginning of period, Shares	298,528	269,063
Granted, Shares	131,290	105,410
Vested, Shares	(50,810)	(69,030)
Forfeited, Shares	(10,585)	(3,005)
Outstanding, end of period, Shares	368,423	302,438
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 83.45	$ 73.91
Granted, Weighted Average Grant Date Fair Value	110.60	91.18
Vested, Weighted Average Grant Date Fair Value	70.24	58.72
Forfeited, Weighted Average Grant Date Fair Value	87.47	82.75
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 94.83	$ 83.31
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, beginning of period, Shares	102,586	115,057
Granted, Shares	29,715	54,751
Vested, Shares	(14,032)	(17,042)
Forfeited, Shares	(7,625)	(2,180)
Outstanding, end of period, Shares	110,644	150,586
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 81.21	$ 78.82
Granted, Weighted Average Grant Date Fair Value	116.82	78.76
Vested, Weighted Average Grant Date Fair Value	70.24	57.99
Forfeited, Weighted Average Grant Date Fair Value	81.91	95.36
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 92.12	$ 80.92