Stock-Based Compensation - Summary of Employee and Director Share Grant Activity (Details) - Restricted Stock - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Grants
Balance at the beginning of the period (in shares)	299,585	318,409	291,749
Granted (in shares)	95,392	119,931	174,558
Released (in shares)	(120,323)	(128,808)	(129,808)
Cancelled (in shares)	(8,179)	(9,947)	(18,090)
Balance at the end of the period (in shares)	266,475	299,585	318,409
Weighted-Average Grant-Date Fair
Balance at the beginning of the period (in dollars per share)	$ 70.57	$ 55.75	$ 37.73
Granted (in dollars per share)	74.49	87.61	69.18
Released (in dollars per share)	63.95	50.05	34.86
Cancelled (in dollars per share)	77.25	67.36	44.54
Balance at the end of the period (in dollars per share)	$ 74.76	$ 70.57	$ 55.75