Share-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Grant Date Fair Value Assumptions [Abstract]
Share-based compensation	$ 9,902,000	$ 9,543,000	$ 9,065,000
Stock Options [Member]
Stock option activity [Roll Forward]
Beginning balance (in shares)	2,741,745
Granted (in shares)	155,225
Exercised (in shares)	343,237	543,028	441,833
Forfeited (in shares)	2,850
Ending balance (in shares)	2,550,883	2,741,745
Exercisable (In Shares)	1,800,183
Stock option weighted average exercise price [Abstract]
Beginning balance (in dollars per share)	$ 31.50
Granted (in dollars per share)	80.95
Exercised (in dollars per share)	30.12
Forfeited (in dollars per share)	28.47
Ending balance (in dollars per share)	34.70	$ 31.50
Exercisable at end of period (in dollars per share)	$ 24.19
Weighted Average Remaining Contractual Term [Abstract]
Weighted average remaining contractual term of shares outstanding (in years)	4 years 2 months 19 days
Weighted average remaining contractual term of shares exercisable at end of period (in years)	2 years 11 months 8 days
Aggregate Intrinsic Value [Abstract]
Ending balance	$ 177,637,707
Exercisable at end of period	$ 144,276,703
Cash Proceeds and Tax Benefits [Abstract]
Exercised (in shares)	343,237	543,028	441,833
Cash proceeds	$ 10,340,000	$ 17,137,000	$ 12,451,000
Intrinsic value of options exercised	21,094,000	22,676,000	13,132,000
Tax benefits realized	$ 7,891,000	$ 8,326,000	$ 5,018,000
Grant Date Fair Value Assumptions [Abstract]
Expected volatility (in hundredths)	29.70%	33.80%	37.70%
Expected term (in years)	7 years 1 month 6 days	7 years 2 months 12 days	7 years 2 months 12 days
Risk-free interest rate (in hundredths)	1.75%	1.95%	2.26%
Expected dividend yield (in hundredths)	1.50%	1.50%	1.50%
Grant date fair value (in dollars per share)	$ 22.86	$ 22.57	$ 21.07
Share-based compensation	$ 3,735,000	$ 3,688,000	$ 3,632,000
Recognized tax benefits	1,409,000	1,331,000	1,388,000
Unamortized compensation expense	$ 3,534,000
Expense recognition over weighted average period (in years)	1 year 8 months 2 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant	1,278,350
Grant Date Fair Value Assumptions [Abstract]
Share-based compensation	$ 5,993,000	$ 5,513,000	$ 5,203,000
Unamortized compensation expense	$ 3,822,000
Expense recognition over weighted average period (in years)	1 year 5 months 14 days
Restricted stock awards [Roll Forward]
Beginning balance (in shares)	301,760
Granted (at market price) (in shares)	80,579
Vested (in shares)	95,420	147,619	212,794
Forfeited (in shares)	1,900
Ending balance (in shares)	285,019	301,760
Restricted stock awards weighted average grant date fair value [Abstract]
Beginning balance (in dollars per share)	$ 51.18
Granted (at market price) (in dollars per share)	83.34
Vested (in dollars per share)	38.70
Forfeited (in dollars per share)	69.55
Ending balance (in dollars per share)	$ 64.33	$ 51.18
Vested (in shares)	95,420	147,619	212,794
Fair value of restricted stock awards vested	$ 7,960,000	$ 10,182,000	$ 12,354,000
ESPP Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares purchased under the ESPP (in shares)	8,649	22,555	22,508
Grant Date Fair Value of most recent ESPP Purchase (per share)	$ 20.42
Grant Date Fair Value Assumptions [Abstract]
Share-based compensation	$ 173,000	$ 342,000	$ 230,000
LTIP 2007 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares allocated for issuance (in shares)	9,315,000
Shares available for grant	4,808,577
LTIP 2002 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares allocated for issuance (in shares)	2,700,000
Number of shares originally allocated for issuance (in shares)	1,575,000
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares allocated for issuance (in shares)	956,250
Discounted percentage rate offered under the employee stock purchase plan (in hundredths)	85.00%