Employee Benefit Plans - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) entrant	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) entrant	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of new entrants after pension plans were frozen | entrant	0		0
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net period benefit costs (less than)	$ 100,000	$ 100,000	$ 200,000	$ 100,000
Expected employer contributions for current year	100,000		100,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net period benefit costs (less than)	200,000	1,200,000	800,000	1,200,000
Employer contributions to the pension plans	$ 0	$ 1,900,000	$ 0	$ 1,900,000
Scenario, Forecast | Minimum
Defined Benefit Plan Disclosure [Line Items]
Unrecognized actuarial losses					$ 17,000,000
Scenario, Forecast | Minimum | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefits					4,000,000
Scenario, Forecast | Maximum
Defined Benefit Plan Disclosure [Line Items]
Unrecognized actuarial losses					19,000,000
Scenario, Forecast | Maximum | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefits					$ 6,000,000