Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income for common stock	$ 217,882	$ 201,774	$ 165,297
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) benefits of $(10,024), $3,468 and $2,886 for 2019, 2018 and 2017, respectively	27,382	(9,472)	(4,370)
Reclassification adjustment for net realized gains included in net income, net of taxes of $175, nil and nil for 2019, 2018 and 2017, respectively	(478)	0	0
Derivatives qualified as cash flow hedges:
Unrealized interest rate hedging losses, net of tax benefit of $409, $151 and nil for 2019, 2018 and 2017, respectively	(1,177)	(436)	0
Reclassification adjustment to net income, net of tax benefits of nil, nil and $289 for 2019, 2018 and 2017, respectively	0	0	454
Retirement benefit plans:
Net gains (losses) arising during the period, net of (taxes) benefits of $(3,892), $9,810 and $(41,129) for 2019, 2018 and 2017, respectively	10,914	(28,101)	65,531
Adjustment for amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits of $3,512, $7,317 and $10,041 for 2019, 2018 and 2017, respectively	10,107	21,015	15,737
Reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, net of (taxes) benefits of $(5,610), $2,887 and $(49,523) for 2019, 2018 and 2017, respectively	(16,177)	8,325	(78,724)
Other comprehensive income (loss), net of taxes	30,571	(8,669)	(1,372)
Comprehensive income	$ 248,453	$ 193,105	$ 163,925