Share-Based Compensation - Weighted average assumptions used in valuing options - (Details) - USD ($)		3 Months Ended
	Jan. 21, 2016	Mar. 31, 2017	Mar. 31, 2016
Stock Options
Weighted-average assumptions
Dividend yield (as a percent)		1.40%	1.60%
Expected life		8 years 6 months	8 years 6 months
Risk-free interest rate (as a percent)		2.43%	1.90%
Additional disclosures
Total unrecognized compensation cost related to non vested stock option grants		$ 1,900,000
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 10 months 17 days
Total fair value of shares vested during the period		$ 578,000
Stock Options | Minimum
Weighted-average assumptions
Expected volatility (as a percent)		37.20%	40.60%
Restricted Stock.
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized		2 years 7 months 21 days
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)		183,014
Granted (in shares)		11,273
Vested (in shares)		(44,011)
Nonvested at the end of the period (in shares)		150,276
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)		$ 51.88
Granted (in dollars per share)		91.22
Vested (in dollars per share)		43.49
Nonvested at the end of the period (in dollars per share)		$ 57.29
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted		$ 5,300,000
Total fair value of shares vested during the period		$ 2,000,000
Restricted Stock. | Employees | Minimum
Additional disclosures
Vesting period		4 years
Restricted Stock. | Non-employee directors | Maximum
Additional disclosures
Vesting period		12 months
Restricted Stock Units
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized		2 years 2 months 12 days
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)		107,876
Granted (in shares)		61,193
Forfeited (in shares)		(3,951)
Nonvested at the end of the period (in shares)		165,118
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)		$ 66.37
Granted (in dollars per share)		89.40
Forfeited (in dollars per share)		63.93
Nonvested at the end of the period (in dollars per share)		$ 74.96
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted		$ 6,200,000
Total fair value of shares vested during the period		$ 2,300,000
Performance period		3 years
Target RSU award level (as a percent)		80.00%
Employee Stock Purchase Plan
Number Of Stock Issued Pursuant To Restricted Stock Units	57,455