Capital Transactions - Stock Compensation (Details) - USD ($)		12 Months Ended
	May 04, 2010	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
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		$ 10,580,000	$ 7,568,000	$ 5,368,000
Director
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		22,617	42,624	36,950
Granted (in dollars per share)		$ 35.81	$ 14.78	$ 14.05
Number of shares
Granted (in shares)		22,617	42,624	36,950
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)		$ 35.81	$ 14.78	$ 14.05
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%	150.00%
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%
Stock options and restricted stock
Compensation expenses
Total unrecognized compensation cost					$ 10,600,000
Unrecognized compensation cost recognized over a weighted average period (in years)		1 year 8 months 12 days
Employee stock option
Share-Based Compensation
Options outstanding (in shares)		819,981	1,064,429	2,888,074	458,103	819,981
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)		819,981	1,064,429	2,888,074
Granted (in shares)			206,021	267,433
Exercised (in shares)		(356,209)	(351,296)	(544,917)
Cancelled (in shares)		(5,669)	(99,173)	(1,546,161)
Outstanding at the end of the period (in shares)		458,103	819,981	1,064,429
Exercisable (in shares)					254,192	263,851
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 13.58	$ 13.64	$ 16.46
Granted (in dollars per share)			14.90	13.63
Exercised (in dollars per share)		13.36	14.17	14.68
Cancelled (in dollars per share)		14.33	14.90	18.53
Outstanding at the end of the period (in dollars per share)		$ 13.73	$ 13.58	$ 13.64
Exercisable (in dollars per share)					$ 13.17	$ 13.76
Weighted Average Remaining Contractual Term
Outstanding, Weighted Average Remaining Contractual Term (in years)		4 years	4 years 4 months 24 days
Exercisable, Weighted Average Remaining Contractual Term (in years)		3 years 4 months 24 days	2 years 9 months 18 days
Aggregate Intrinsic Value
Average intrinsic value of shares outstanding					$ 18,034,100	$ 18,756,900
Average intrinsic value of shares exercisable					$ 10,150,800	$ 5,985,500
Total intrinsic value of options exercised		$ 9,940,000	$ 4,250,000	$ 1,800,000
Non-vested stock options
Non-vested shares at beginning of year (in shares)		556,130
Granted (in shares)			206,021	267,433
Vested (in shares)		(346,550)
Cancelled (in shares)		(5,669)
Non-vested shares at end of year (in shares)		203,911	556,130
Weighted Average Grant-Date Fair Value
Non-vested shares at beginning of year (in dollars per share)		$ 4.86
Vested (in dollars per share)		4.67
Cancelled (in dollars per share)		5.14
Non-vested shares at end of year (in dollars per share)		$ 5.17	$ 4.86
Performance share units
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		119,315	183,577	222,583
Granted (in dollars per share)		$ 35.81	$ 14.89	$ 13.61
Performance period		3 years
Number of shares
Nonvested shares at the beginning of the period (in shares)		363,993	202,829
Granted (in shares)		119,315	183,577	222,583
Cancelled (in shares)		(14,732)	(22,413)	(19,754)
Nonvested shares at the end of the period (in shares)		468,576	363,993	202,829
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share)		$ 14.23	$ 13.62
Granted (in dollars per share)		35.81	14.89	$ 13.61
Cancelled (in dollars per share)		15.00	14.16	13.51
Nonvested shares at the end of the period (in dollars per share)		$ 19.70	$ 14.23	$ 13.62
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)		160,137	384,148	408,163
Granted (in dollars per share)		$ 35.81	$ 14.81	$ 13.57
Number of shares
Nonvested shares at the beginning of the period (in shares)		926,931	809,299	723,176
Granted (in shares)		160,137	384,148	408,163
Vested (in shares)		(230,903)	(215,146)	(215,856)
Cancelled (in shares)		(40,575)	(51,370)	(106,184)
Nonvested shares at the end of the period (in shares)		815,590	926,931	809,299
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share)		$ 13.65	$ 13.13	$ 12.70
Granted (in dollars per share)		35.81	14.81	13.57
Vested (in dollars per share)		12.01	13.29	13.06
Cancelled (in dollars per share)		15.78	13.72	13.52
Nonvested shares at the end of the period (in dollars per share)		$ 18.35	$ 13.65	$ 13.13
Long Term Incentive Plan 2010
Share-Based Compensation
Number of shares authorized	5,150,000
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Vesting period			3 years	3 years
Long Term Incentive Plan 2010 | Employee stock option
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
Expected annual dividend rate (as a percent)			1.08%	1.18%
Risk-free interest rate (as a percent)			1.15%	1.62%
Average expected life (years)			5 years 8 months 12 days	5 years 8 months 12 days
Expected volatility of common stock (as a percent)			0.412%	0.401%
Forfeiture rate (as a percent)			0.00%	0.00%
Weighted average fair value of option grants			$ 5.27	$ 4.75
Number of Options
Granted (in shares)			206,021	267,433
Non-vested stock options
Granted (in shares)			206,021	267,433
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)			$ 5.27	$ 4.75
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)		$ 35.81
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)		$ 35.81
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)		160,137
Number of shares
Granted (in shares)		160,137