Incentive Plan - Summary of Stock Options (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Number of Options Granted
Outstanding, beginning of period (in shares) | shares	323,770
Options granted (in shares) | shares	333,484
Options forfeited (in shares) | shares	(2,158)
Options exercised (in shares) | shares	0
Options expired (in shares) | shares	(1,080)
Outstanding, end of period (in shares) | shares	654,016
Exercisable, end of period (in shares) | shares	109,019
Vested or expected to vest, end of period (in shares) | shares	544,997
Weighted Average Exercise Price
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 36.55
Options granted (in dollars per share) | $ / shares	41.80
Options forfeited (in dollars per share) | $ / shares	36.55
Options exercised (in dollars per share) | $ / shares	0.00
Options expired (in dollars per share) | $ / shares	36.55
Outstanding, end of period (in dollars per share) | $ / shares	39.23
Exercisable, end of period (in dollars per share) | $ / shares	36.55
Vested or expected to vest, end of period (in dollars per share) | $ / shares	$ 39.76
Weighted Average Remaining Contractual Term (Years)
Outstanding, end of period	9 years 26 days
Exercisable, end of period	8 years 11 months 23 days
Vested or expected to vest, end of period	9 years 1 month 2 days
Aggregate Intrinsic Value
Outstanding, end of period | $	$ 0
Exercisable, end of period | $	0
Vested or expected to vest, end of period | $	$ 0