Stock-Based Compensation Plans - Restricted stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Apr. 01, 2018	May 24, 2007
2007 Stock Incentive Plan
Aggregate Grant Date Fair Value
Number of shares authorized (in shares)				2,800,000
2007 Stock Incentive Plan | Restricted Stock
Number Outstanding
Balance at the beginning of the period (in shares)	417,890	328,122
Shares granted (in shares)	279,400	270,454
Shares forfeited (in shares)	(3,866)	(9,900)
Shares vested (in shares)	(187,957)	(170,786)
Balance at the end of the period (in shares)	505,467	417,890
Weighted Average Grant Date Fair Value
Balance at the beginning of the period (in dollars per share)	$ 30.54	$ 23.49
Shares granted (in dollars per share)	42.28	33.91
Shares cancelled (in dollars per share)	30.43	32.06
Shares vested (in dollars per share)	28.74	22.25
Balance at the end of the period (in dollars per share)	$ 37.70	$ 30.54
Aggregate Grant Date Fair Value
Balance at beginning of period	$ 12,763,000	$ 7,707,000
Granted	11,813,000	9,172,000
Vested	(5,402,000)	(3,799,000)
Forfeited	(118,000)	(317,000)
Balance at end of period	$ 19,056,000	$ 12,763,000
Remaining average vesting period for recognition of unrecognized compensation expense	1 year 8 months 12 days
Unrecognized compensation expense	$ 12,600,000
Intrinsic value of unvested shares	$ 29,800,000
Number of shares awarded (in shares)	279,400
Employee Stock Purchase Plan
Weighted Average Grant Date Fair Value
Shares granted (in dollars per share)	$ 23.20	$ 9.42
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)	13,193	11,132