Fair Value Measurements - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related allowance	$ 10,800,000	$ 14,000,000	$ 11,600,000
Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	22,200,000	(25,800,000)
Carrying value of assets to be disposed of	800,000	2,400,000
Write down of assets to be disposed of	0	1,100,000
Fair value of assets to be disposed of	800,000	1,300,000
Fair Value Measured on a Non-recurring Basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	32,600,000	39,300,000
Related allowance	10,700,000	14,000,000
Partial loan charge-off	11,400,000	11,900,000
Carrying Amount | Fair Value Measured on a Non-recurring Basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	54,700,000	65,200,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	32,600,000	39,300,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 32,600,000	$ 39,300,000