Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Detail of regulatory liabilities
Regulatory liabilities, current	$ 91,173	$ 99,899
Regulatory liabilities, non-current	940,106	948,916
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	321,732	279,976
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	37,943	29,899
Regulatory liabilities, non-current	189,959	223,145
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	32,725	32,662
Regulatory liabilities, non-current	107,764	123,913
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	4,315	4,368
Regulatory liabilities, non-current	107,153	108,827
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,565	1,771
Regulatory liabilities, non-current	68,583	70,898
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	71,996	71,726
Renewable energy standard (c)
Detail of regulatory liabilities
Regulatory liabilities, current	11,519	26,809
Regulatory liabilities, non-current	0	0
Demand side management (c)
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	19,921	20,472
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	16,092	15,287
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,063	2,063
Regulatory liabilities, non-current	7,851	8,895
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	20,894	18,248
Other
Detail of regulatory liabilities
Regulatory liabilities, current	43	2,327
Regulatory liabilities, non-current	$ 8,161	$ 7,529