Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income for common stock	$ 246,166	$ 197,824	$ 217,882
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 $(18,903), $7,008 and $10,024 for 2021, 2020 and 2019, respectively	(51,636)	19,143	27,382
Reclassification adjustment for net realized gains included in net income, net of taxes of $(142), $(599) and $(175) for 2021, 2020 and 2019, respectively	(387)	(1,638)	(478)
Derivatives qualified as cash flow hedges:
Unrealized interest rate hedging losses, net of taxes of $(108), $(607) and $(409) for 2021, 2020 and 2019, respectively	(312)	(1,750)	(1,177)
Reclassification adjustment to net income, net of taxes of $13, nil and nil for 2021, 2020 and 2019, respectively	37
Retirement benefit plans:
Net gains (losses) arising during the period, net of taxes of $53,068, $(20,907) and $3,892 for 2021, 2020 and 2019, respectively	153,121	(60,529)	10,914
Adjustment for amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of taxes of $6,665, $8,247 and $3,512 for 2021, 2020 and 2019, respectively	19,253	23,689	10,107
Reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of taxes of $(59,429), $13,825 and $(5,610) for 2021, 2020 and 2019, respectively	(171,345)	39,860	(16,177)
Other comprehensive loss, net of tax benefits	(51,269)	18,775	30,571
Comprehensive income	$ 194,897	$ 216,599	$ 248,453