Share-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Grant Date Fair Value Assumptions [Abstract]
Share-based compensation	$ 9,543,000	$ 9,065,000	$ 8,150,000
Stock Options [Member]
Stock option activity [Roll Forward]
Beginning balance (in shares)	3,112,098
Granted (in shares)	176,375
Exercised (in shares)	543,028	441,833	780,485
Forfeited (in shares)	3,700
Ending balance (in shares)	2,741,745	3,112,098
Exercisable (In Shares)	1,820,919
Stock option weighted average exercise price [Abstract]
Beginning balance (in dollars per share)	$ 29.35
Granted (in dollars per share)	69.85
Exercised (in dollars per share)	31.56
Forfeited (in dollars per share)	35.82
Ending balance (in dollars per share)	31.50	$ 29.35
Exercisable at end of period (in dollars per share)	$ 23.86
Weighted Average Remaining Contractual Term [Abstract]
Weighted average remaining contractual term of shares outstanding (in years)	4 years 6 months 29 days
Weighted average remaining contractual term of shares exercisable at end of period (in years)	3 years 3 months 14 days
Aggregate Intrinsic Value [Abstract]
Ending balance	$ 135,100,939
Exercisable at end of period	$ 103,643,264
Cash Proceeds and Tax Benefits [Abstract]
Exercised (in shares)	543,028	441,833	780,485
Cash proceeds	$ 17,137,000	$ 12,451,000	$ 20,323,000
Intrinsic value of options exercised	22,676,000	13,132,000	19,273,000
Tax benefits realized	$ 8,326,000	$ 5,018,000	$ 7,364,000
Grant Date Fair Value Assumptions [Abstract]
Expected volatility (in hundredths)	33.80%	37.70%	38.70%
Expected term (in years)	7 years 2 months 12 days	7 years 2 months 12 days	7 years 6 months 10 days
Risk-free interest rate (in hundredths)	1.95%	2.26%	1.51%
Expected dividend yield (in hundredths)	1.50%	1.50%	1.50%
Grant date fair value (in dollars per share)	$ 22.57	$ 21.07	$ 16.42
Share-based compensation	$ 3,688,000	$ 3,632,000	$ 3,746,000
Recognized tax benefits	1,331,000	1,388,000	1,431,000
Unamortized compensation expense	$ 3,723,000
Expense recognition over weighted average period (in years)	1 year 7 months 26 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant	357,029
Grant Date Fair Value Assumptions [Abstract]
Share-based compensation	$ 5,513,000	$ 5,203,000	$ 4,103,000
Unamortized compensation expense	$ 3,435,000
Expense recognition over weighted average period (in years)	1 year 6 months 28 days
Restricted stock awards [Roll Forward]
Beginning balance (in shares)	365,859
Granted (at market price) (in shares)	89,580
Vested (in shares)	147,619	212,794	102,536
Forfeited (in shares)	6,060
Ending balance (in shares)	301,760	365,859
Restricted stock awards weighted average grant date fair value [Abstract]
Beginning balance (in dollars per share)	$ 37.47
Granted (at market price) (in dollars per share)	69.10
Vested (in dollars per share)	27.89
Forfeited (in dollars per share)	55.56
Ending balance (in dollars per share)	$ 51.18	$ 37.47
Vested (in shares)	147,619	212,794	102,536
Fair value of restricted stock awards vested	$ 10,182,000	$ 12,354,000	$ 4,544,000
ESPP Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares purchased under the ESPP (in shares)	22,555	22,508	26,644
Grant Date Fair Value of most recent ESPP Purchase (per share)	$ 16.29
Grant Date Fair Value Assumptions [Abstract]
Share-based compensation	$ 342,000	$ 230,000	$ 301,000
LTIP 2007 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares allocated for issuance (in shares)	5,415,000
Number of shares originally allocated for issuance (in shares)	1,515,000
Shares available for grant	1,140,981
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
Director plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares allocated for issuance (in shares)	1,350,000
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%