Leases - Capital Leases (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) renewal_option	Dec. 31, 2016 USD ($)
Capital Leased Assets [Line Items]
Capital leases, number of renewal options | renewal_option	2
Capital leases, renewal option period one	10 years
Capital leases, renewal option period two	5 years
Minimum capital lease payment obligations	$ 42,019,000
Fiber facilities
Capital Leased Assets [Line Items]
Capital leases, renewal option period one	25 years
Capital lease assets	$ 16,100,000	$ 15,900,000
Minimum capital lease payment obligations	$ 2,300,000
Capital leases, term of lease	25 years
Substation equipment
Capital Leased Assets [Line Items]
Capital lease assets	$ 515,000	$ 930,000
Minimum capital lease payment obligations	$ 0
Capital leases, term of lease	39 years