Financial Instruments and Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt principal amount			$ 180,000,000.0
Debt interest rate			4.80%
Senior notes held by special purpose entity	$ 176,595,000	$ 176,537,000
Carrying Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes held by special purpose entity	176,595,000	$ 176,537,000
AgReserves Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Note received in sale of real estate			$ 200,000,000
Maturity of installment note			15 years
Subsidiaries | Notes Receivable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Issue price of senior secured notes			98.50%
Special Purpose Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Time deposits held by special purpose entities	200,000,000.0
U.S. Treasury securities	7,200,000
Cash held by special purpose entities	$ 400,000
Special Purpose Entities | Notes Receivable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt interest rate	4.00%
Northwest Florida Timber Finance, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes held by special purpose entity	$ 176,600,000
Unamortized discount and debt issuance costs	$ 3,400,000
Short-term bond | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assumptions, expected term	0 years	0 years
Short-term bond | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assumptions, expected term	3 years	3 years