Incentive Stock Plans - Schedule of Performance Share Activity (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted Average Grant Date Fair Value
Interest (b)	[1]	$ 33,120	$ 36,041	$ 36,289
Performance Shares
Number of Units
Beginning Balance (in shares)		329,239
Granted (in shares)		106,577
Units Distributed (in shares)		(102,523)
Other Cancellations/Adjustments (in shares)		(11)
Ending Balance (in shares)		333,282	329,239
Weighted Average Grant Date Fair Value
Beginning Balance (in dollars per share)		$ 30.21
Weighted average price of restricted shares granted (in dollars per share)		40.27	$ 32.17	$ 28.79
Units Distributed (in dollars per share)		29.62
Other Cancellations/Adjustments (in dollars per share)		30.24
Ending Balance (in dollars per share)		$ 33.60	$ 30.21

[1] (b)Interest paid is presented net of patronage payments received of $4.1 million and $3.0 million for the years ended December 31, 2018 and December 31, 2017, respectively. For additional information on patronage payments, see Note 5 — Debt.