Capital Transactions - Stock Compensation (Details)		12 Months Ended
	May 04, 2010 shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) installment $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Performance period		90 days
Compensation expenses
Stock based compensation expense | $		$ 13,105,000	$ 10,580,000	$ 7,568,000
Director
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		15,165	22,617
Granted (in dollars per share) | $ / shares		$ 53.40	$ 35.81	$ 14.78
Number of shares
Granted (in shares)		15,165	22,617
Weighted Average Grant-Date Fair Value
Granted (in dollars per share) | $ / shares		$ 53.40	$ 35.81	$ 14.78
Number of Options
Granted (in shares)				42,624
Non-vested stock options
Granted (in shares)				42,624
Stock options and restricted stock
Compensation expenses
Total unrecognized compensation cost | $					$ 12,700,000
Unrecognized compensation cost recognized over a weighted average period (in years)		1 year 8 months 12 days
Employee stock options
Share-Based Compensation
Options outstanding (in shares)		458,103	819,981	1,064,429	300,580	458,103
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Expected annual dividend rate (as a percent)				1.08%
Risk-free interest rate (as a percent)				1.15%
Average expected life (years)				5 years 8 months 12 days
Expected volatility of common stock (as a percent)				0.412%
Forfeiture rate (as a percent)				0.00%
Weighted average fair value of option grants | $ / shares				$ 5.27
Compensation expenses
Period from grant date after which stock options become exercisable, minimum (in months)		6 months
Period from grant date within which incentive stock options are exercisable, maximum (in years)		7 years
Number of Options
Outstanding at the beginning of the period (in shares)		458,103	819,981	1,064,429
Granted (in shares)				206,021
Exercised (in shares)		(157,523)	(356,209)	(351,296)
Cancelled (in shares)			(5,669)	(99,173)
Outstanding at the end of the period (in shares)		300,580	458,103	819,981
Exercisable (in shares)					235,672	254,192
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares		$ 13.73	$ 13.58	$ 13.64
Granted (in dollars per share) | $ / shares				14.90
Exercised (in dollars per share) | $ / shares		13.80	13.36	14.17
Cancelled (in dollars per share) | $ / shares			14.33	14.90
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 13.70	$ 13.73	$ 13.58
Exercisable (in dollars per share) | $ / shares					$ 13.36	$ 13.17
Weighted Average Remaining Contractual Term
Outstanding, Weighted Average Remaining Contractual Term (in years)		3 years	4 years
Exercisable, Weighted Average Remaining Contractual Term (in years)		2 years 8 months 12 days	3 years 4 months 24 days
Aggregate Intrinsic Value
Average intrinsic value of shares outstanding | $					$ 9,249,400	$ 18,034,100
Average intrinsic value of shares exercisable | $					$ 7,330,700	$ 10,150,800
Total intrinsic value of options exercised | $		$ 7,100,000	$ 9,940,000	$ 4,250,000
Non-vested stock options
Non-vested shares at beginning of year (in shares)		203,911
Granted (in shares)				206,021
Vested (in shares)		(139,003)
Non-vested shares at end of year (in shares)		64,908	203,911
Weighted Average Grant-Date Fair Value
Non-vested shares at beginning of year (in dollars per share) | $ / shares		$ 5.17
Granted (in dollars per share) | $ / shares				$ 5.27
Vested (in dollars per share) | $ / shares		5.10
Non-vested shares at end of year (in dollars per share) | $ / shares		$ 5.32	$ 5.17
Performance stock units
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		90,264	119,315	183,577
Granted (in dollars per share) | $ / shares		$ 53.41	$ 35.81	$ 14.89
Performance period		3 years
Number of shares
Nonvested sharess at the beginning of the period (in shares)		468,576	363,993	202,829
Granted (in shares)		90,264	119,315	183,577
Additional shares awarded from the 2015 grant		92,335
Vested (in shares)		(277,029)
Cancelled (in shares)		(3,229)	(14,732)	(22,413)
Nonvested shares at the end of the period (in shares)		370,917	468,576	363,993
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share) | $ / shares		$ 19.70	$ 14.23	$ 13.62
Granted (in dollars per share) | $ / shares		53.41	35.81	14.89
Additional shares awarded from the 2015 grant (in dollars per share) | $ / shares		13.62
Vested (in dollars per share) | $ / shares		13.62
Cancelled (in dollars per share) | $ / shares		30.09	15.00	14.16
Nonvested shares at the end of the period (in dollars per share) | $ / shares		$ 30.84	$ 19.70	$ 14.23
Performance stock units | Maximum
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Award percentage		200.00%	200.00%	150.00%
Performance stock units | Minimum
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Award percentage		0.00%	0.00%	0.00%
Restricted stock units
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		115,044	160,137	384,148
Granted (in dollars per share) | $ / shares		$ 53.40	$ 35.81	$ 14.81
Number of shares
Nonvested sharess at the beginning of the period (in shares)		815,590	926,931	809,299
Granted (in shares)		115,044	160,137	384,148
Vested (in shares)		(330,580)	(230,903)	(215,146)
Cancelled (in shares)		(24,273)	(40,575)	(51,370)
Nonvested shares at the end of the period (in shares)		575,781	815,590	926,931
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share) | $ / shares		$ 18.35	$ 13.65	$ 13.13
Granted (in dollars per share) | $ / shares		53.40	35.81	14.81
Vested (in dollars per share) | $ / shares		13.57	12.01	13.29
Cancelled (in dollars per share) | $ / shares		27.77	15.78	13.72
Nonvested shares at the end of the period (in dollars per share) | $ / shares		$ 27.71	$ 18.35	$ 13.65
Long Term Incentive Plan 2010
Share-Based Compensation
Number of shares authorized	5,150,000
Shares remaining available for future issuance					2,000,000
Long Term Incentive Plan 2010 | Employee stock options
Share-Based Compensation
Minimum incentive stock option exercise price, expressed as a percentage of common stock grant date market value	100.00%
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Number of equal installments to vest | installment				3
Vesting period				3 years
Number of Options
Granted (in shares)				206,021
Non-vested stock options
Granted (in shares)				206,021
Long Term Incentive Plan 2010 | Restricted stock units
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Vesting period		3 years
Granted (in dollars per share) | $ / shares		$ 53.40
Weighted Average Grant-Date Fair Value
Granted (in dollars per share) | $ / shares		$ 53.40
Long Term Incentive Plan 2010 | Restricted stock units | Director
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		115,044
Number of shares
Granted (in shares)		115,044