Capital Transactions - Stock Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	May 04, 2010
Share-Based Compensation
Performance period	90 days
Compensation expenses
Stock based compensation expense	$ 6,802,000	$ 10,274,000	$ 13,105,000
Employee severance related costs	7,900,000
Forfeiture credit partially offset	$ 4,500,000
Director
Share-Based Compensation
Granted (in shares)	14,643	18,576
Granted (in dollars per share)	$ 61.45	$ 48.45	$ 53.40
Number of shares
Granted (in shares)	14,643	18,576
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)	$ 61.45	$ 48.45	$ 53.40
Number of Options
Granted (in shares)			15,165
Non-vested stock options
Granted (in shares)			15,165
Restricted stock grants and performance stock units
Compensation expenses
Total unrecognized compensation cost				$ 9,200,000
Unrecognized compensation cost recognized over a weighted average period (in years)	1 month 21 days
Employee stock options
Share-Based Compensation
Options outstanding (in shares)	57,653	300,580	458,103	57,653	60,231
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)	60,231	300,580	458,103
Exercised (in shares)	(2,578)	(232,514)	(157,523)
Cancelled (in shares)		(7,835)
Outstanding at the end of the period (in shares)	57,653	60,231	300,580
Exercisable (in shares)				57,653	60,231
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 14.74	$ 13.70	$ 13.73
Exercised (in dollars per share)	14.78	13.36	13.80
Cancelled (in dollars per share)		15.86
Outstanding at the end of the period (in dollars per share)	$ 14.74	$ 14.74	$ 13.70
Exercisable (in dollars per share)				$ 14.74	$ 14.74
Weighted Average Remaining Contractual Term
Outstanding, Weighted Average Remaining Contractual Term (in years)	2 years 1 month 6 days	3 years 1 month 6 days
Exercisable, Weighted Average Remaining Contractual Term (in years)	2 years 1 month 6 days	3 years 1 month 6 days
Aggregate Intrinsic Value
Average intrinsic value of shares outstanding				$ 1,474,500	$ 3,005,100
Average intrinsic value of shares exercisable				$ 1,474,500	$ 3,005,100
Total intrinsic value of options exercised	$ 100,000	$ 10,500,000	$ 7,100,000
Performance stock units
Share-Based Compensation
Granted (in shares)	69,132	87,864	90,264
Granted (in dollars per share)	$ 61.45	$ 48.81	$ 53.41
Performance period	3 years
Number of shares
Nonvested sharess at the beginning of the period (in shares)	233,571	370,917	468,576
Granted (in shares)	69,132	87,864	90,264
Additional shares awarded from the 2015 grant	83,042	67,853	92,335
Vested (in shares)	(166,084)	(203,582)	(277,029)
Cancelled (in shares)	(23,052)	(89,481)	(3,229)
Nonvested shares at the end of the period (in shares)	196,609	233,571	370,917
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share)	$ 45.44	$ 30.84	$ 19.70
Granted (in dollars per share)	61.45	48.81	53.41
Additional shares awarded from the 2015 grant (in dollars per share)	35.81	14.80	13.62
Vested (in dollars per share)	35.81	14.80	13.62
Cancelled (in dollars per share)	53.66	34.70	30.09
Nonvested shares at the end of the period (in dollars per share)	$ 54.17	$ 45.44	$ 30.84
Performance stock units | Maximum
Share-Based Compensation
Award percentage	200.00%	200.00%	200.00%
Performance stock units | Minimum
Share-Based Compensation
Award percentage	0.00%	0.00%	0.00%
Restricted stock units
Share-Based Compensation
Granted (in shares)	82,505	104,120	115,044
Granted (in dollars per share)	$ 61.45	$ 48.65	$ 53.40
Number of shares
Nonvested sharess at the beginning of the period (in shares)	284,686	575,781	815,590
Granted (in shares)	82,505	104,120	115,044
Vested (in shares)	(103,231)	(251,853)	(330,580)
Cancelled (in shares)	(25,983)	(143,362)	(24,273)
Nonvested shares at the end of the period (in shares)	237,977	284,686	575,781
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share)	$ 45.21	$ 27.71	$ 18.35
Granted (in dollars per share)	61.45	48.65	53.40
Vested (in dollars per share)	35.81	14.79	13.57
Cancelled (in dollars per share)	52.31	30.85	27.77
Nonvested shares at the end of the period (in dollars per share)	$ 54.15	$ 45.21	$ 27.71
Long Term Incentive Plan 2010
Share-Based Compensation
Number of shares authorized						4,500,000
Shares remaining available for future issuance				4,100,000
Long Term Incentive Plan 2010 | Restricted stock units
Share-Based Compensation
Vesting period	3 years
Granted (in dollars per share)	$ 61.45
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)	$ 61.45
Long Term Incentive Plan 2010 | Restricted stock units | Director
Share-Based Compensation
Granted (in shares)	82,505
Number of shares
Granted (in shares)	82,505