Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income for common stock	$ 197,824	$ 217,882	$ 201,774
Net unrealized gains (losses) on available-for sale investment securities:
Net unrealized gains (losses) on available-for sale investment securities arising during the period, net of taxes of $7,008, $10,024 and $(3,468) for 2020, 2019 and 2018, respectively	19,143	27,382	(9,472)
Reclassification adjustment for net realized gains included in net income, net of taxes of $(599), $(175) and nil for 2020, 2019 and 2018, respectively	(1,638)	(478)	0
Derivatives qualified as cash flow hedges:
Unrealized interest rate hedging losses, net of taxes of $(607), $(409) and $(151) for 2020, 2019 and 2018, respectively	(1,750)	(1,177)	(436)
Retirement benefit plans:
Net gains (losses) arising during the period, net of taxes of $(20,907), $3,892 and $(9,810) for 2020, 2019 and 2018, respectively	(60,529)	10,914	(28,101)
Adjustment for amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of taxes of $8,247, $3,512 and $7,317 for 2020, 2019 and 2018, respectively	23,689	10,107	21,015
Reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $13,825, $(5,610) and $2,887 for 2020, 2019 and 2018, respectively	39,860	(16,177)	8,325
Other comprehensive income (loss), net of taxes	18,775	30,571	(8,669)
Comprehensive income	$ 216,599	$ 248,453	$ 193,105