Derivative and Hedging - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 31, 2016 USD ($) counterparty	Sep. 30, 2012 USD ($)
Derivative [Line Items]
Other long-term investments	$ 8,193,000	$ 7,369,000
Cost method investments	7,629,000	6,919,000
Interest Expense [Member]
Derivative [Line Items]
Amount reclassified as an increase to interest expense during next twelve months	2,400,000
Interest Rate Swap, September 2012 [Member] | Not Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amount of notional principal interest rate swap				$ 174,600,000.0
Notional amount of interest rate swaps	117,900,000	135,300,000
Interest Rate Swap, May 2016 [Member] | Not Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amount of notional principal interest rate swap			$ 256,600,000.0
Notional amount of interest rate swaps	$ 300,400,000	301,100,000
Number of counterparties | counterparty			3
Percentage hedged	50.00%
CoBank [Member]
Derivative [Line Items]
Cost method investments	$ 6,818,000	$ 6,177,000