CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension and retiree medical benefits:
Net pension and retiree medical benefit gains arising during the period, tax	$ 300	$ (400)	$ (500)
Amortization of losses included in net periodic benefit cost, tax	100	100	0
Derivative instruments:
Net fair value (decrease) increase, tax	0	0	0
Reclassification of losses to net income, tax	300	600	600
Income taxes	47,400	27,200	199,700
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	0	0	$ 0
Gains and Losses on Cash Flow Hedges \| Amounts Reclassified from Accumulated Other Comprehensive Loss
Derivative instruments:
Net fair value (decrease) increase, tax	0	0
Income taxes	300	300
Reclassification from AOCI, Current Period, Tax	0	0
Defined Benefit Pension and Postretirement Items \| Amounts Reclassified from Accumulated Other Comprehensive Loss
Derivative instruments:
Net fair value (decrease) increase, tax	(100)	300
Income taxes	0	0
Reclassification from AOCI, Current Period, Tax	$ 100	$ 100

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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