Years Subject to Examination (Details)	12 Months Ended
	Dec. 31, 2018
Federal
Income Tax Examination
Income tax return jurisdictions and years that remain subject to examination	2015, 2016 and 2017
Montana
Income Tax Examination
Income tax return jurisdictions and years that remain subject to examination	2015, 2016 and 2017
Idaho
Income Tax Examination
Income tax return jurisdictions and years that remain subject to examination	2015, 2016 and 2017
Utah
Income Tax Examination
Income tax return jurisdictions and years that remain subject to examination	2015, 2016 and 2017
Colorado
Income Tax Examination
Income tax return jurisdictions and years that remain subject to examination	2014, 2015, 2016 and 2017
Arizona
Income Tax Examination
Income tax return jurisdictions and years that remain subject to examination	2017