Employee Benefit Plans (Details)		1 Months Ended	12 Months Ended
	Feb. 08, 2018	Feb. 28, 2018	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee stock ownership plan, contribution expense			$ 10,769,000	$ 6,343,000	$ 5,488,000
Deferred compensation expense			$ 2,309,000	1,596,000	1,056,000
Defined Contribution Plan Disclosure [Line Items]
Purchase price of common stock (as percent)			95.00%
Stock split, conversion ratio	2	2
Allocated stock-based compensation expense			$ 11,227,000	$ 11,763,000	$ 5,451,000
Compensation not yet recognized, stock options			0
Compensation not yet recognized, share-based awards other than options			$ 14,714,000
Compensation cost not yet recognized, period for recognition			1 year 8 months 12 days
Options outstanding, intrinsic value			$ 1,187,000
Options exercisable, intrinsic value			$ 1,165,000
Share price (in dollars per share) | $ / shares			$ 24.13
Options, exercises in period | shares			111,728	122,012	127,424
Options, exercises in period, intrinsic value			$ 561,000	$ 1,329,000	$ 2,031,000
Options, grants in period, gross | shares			0	0	116,000
Options, grants in period, weighted average grant date fair value (in dollars per share) | $ / shares					$ 5.82
Employee Stock Purchase Plan 2015
Defined Contribution Plan Disclosure [Line Items]
Maximum annual contributions per employee, amount			$ 25,000
Service period			6 months
Special Employee Stock Purchase Plan 2015
Defined Contribution Plan Disclosure [Line Items]
Maximum annual contributions per employee, amount			$ 10,000
Purchase price of common stock (as percent)			85.00%