Stock-Based Compensation Plans - Restricted Stock Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Apr. 01, 2018	May 31, 2007
2007 Stock Incentive Plan
Aggregate Grant Date Fair Value
Number of shares authorized (in shares)				2,800,000
Employee Stock Purchase Plan
Aggregate Grant Date Fair Value
Number of shares authorized (in shares)			325,000
Maximum percentage of cash compensation allowed to be deducted for the purchase of common stock by eligible employees	10.00%
Maximum number of shares to be purchased by employee in one calendar year (in shares)	1,000
Maximum amount of shares to be purchased by employee in one calendar year	$ 25,000
Purchase price expressed as a percentage of the market price of the common stock on the purchase date or on the date of entry	85.00%
Shares issued (in shares)	11,394	13,193
Restricted Stock
Number Outstanding
Balance at the beginning of the period (in shares)	505,467	417,890
Shares granted (in shares)	319,350	279,400
Shares forfeited (in shares)	0	(3,866)
Shares vested (in shares)	(243,102)	(187,957)
Balance at the end of the period (in shares)	581,715	505,467
Weighted Average Grant Date Fair Value
Balance at the beginning of the period (in dollars per share)	$ 37.70	$ 30.54
Shares granted (in dollars per share)	19.00	42.28
Shares cancelled (in dollars per share)	0	30.43
Shares vested (in dollars per share)	28.35	28.74
Balance at the end of the period (in dollars per share)	$ 31.34	$ 37.70
Aggregate Grant Date Fair Value
Balance at beginning of period	$ 19,056,000	$ 12,763,000
Granted	6,068,000	11,813,000
Forfeited	0	(118,000)
Vested	(6,892,000)	(5,402,000)
Balance at end of period	$ 18,232,000	$ 19,056,000
Restricted Stock | 2007 Stock Incentive Plan
Aggregate Grant Date Fair Value
Remaining average vesting period for recognition of unrecognized compensation expense	1 year 3 months 18 days
Unrecognized compensation expense	$ 9,200,000
Intrinsic value of unvested shares	$ 17,700,000