Stock-Based Compensation - Status of Nonvested Restricted Stock, Stock Grants, Stock Units and Performance Shares (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units, Stock Grants, and Stock Units
Nonvested shares
Balance at the beginning of the period (in shares)	335,259
Granted (in shares)	161,963
Change in performance factor (in shares)	0
Vested (in shares)	(202,327)
Forfeited (in shares)	(3,607)
Balance at the end of the period (in shares)	291,288	335,259
Weighted-Average Grant-Date Fair Value
Balance at the beginning of the period (in dollars per share)	$ 62.04
Granted (in dollars per share)	72.60	$ 67.34	$ 64.12
Change in performance factor (in dollars per share)	0.00
Vested (in dollars per share)	59.19
Forfeited (in dollars per share)	69.58
Balance at the end of the period (in dollars per share)	$ 69.78	$ 62.04
Vested Awards Outstanding at December 31, 2017	89,928
Vested Awards Outstanding at December 31, 2015 (in shares)
Number of nonvested awards to be settled in cash (in shares)	133,373
Performance Shares
Nonvested shares
Balance at the beginning of the period (in shares)	312,724
Granted (in shares)	147,706
Change in performance factor (in shares)	18,266
Vested (in shares)	(164,396)
Forfeited (in shares)	(4,798)
Balance at the end of the period (in shares)	309,502	312,724
Weighted-Average Grant-Date Fair Value
Balance at the beginning of the period (in dollars per share)	$ 65.32
Granted (in dollars per share)	78.99	$ 66.60	$ 64.97
Change in performance factor (in dollars per share)	64.97
Vested (in dollars per share)	63.87
Forfeited (in dollars per share)	69.77
Balance at the end of the period (in dollars per share)	$ 72.46	$ 65.32
Vested Awards Outstanding at December 31, 2017	164,396
Vested Awards Outstanding at December 31, 2015 (in shares)