Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Regulatory Assets [Line Items]
Current regulatory assets	$ 37,504	$ 42,362	$ 49,004
Non-current regulatory assets	348,284	357,530	344,969
Annual tariff rider collection	5,000
Unrealized loss on derivatives
Regulatory Assets [Line Items]
Current regulatory assets	4,625	1,315	3,439
Non-current regulatory assets	3,466	913	474
Gas costs
Regulatory Assets [Line Items]
Current regulatory assets	859	6,830	9,571
Non-current regulatory assets	87	89	1,487
Environmental costs
Regulatory Assets [Line Items]
Current regulatory assets	6,724	9,989	9,610
Non-current regulatory assets	64,008	63,970	65,983
Decoupling
Regulatory Assets [Line Items]
Current regulatory assets	12,136	13,067	14,170
Non-current regulatory assets	1,993	5,860	1,776
Other
Regulatory Assets [Line Items]
Current regulatory assets	13,160	11,161	12,214
Non-current regulatory assets	10,645	14,130	8,786
Pension balancing
Regulatory Assets [Line Items]
Non-current regulatory assets	55,358	50,863	48,761
Income taxes
Regulatory Assets [Line Items]
Non-current regulatory assets	36,591	38,670	40,106
Pension and other postretirement benefit liabilities
Regulatory Assets [Line Items]
Non-current regulatory assets	$ 176,136	$ 183,035	$ 177,596