Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net unrealized gains (losses) on securities arising during the period, tax (benefits)	$ (2,886)	$ (3,763)	$ (1,541)
Reclassification adjustment for net realized gains included in net income, taxes	0	238	0
Foreign currency hedge net unrealized loss, (taxes) benefits	0	179	0
Reclassification adjustment to net income, (taxes) benefits	289	(76)	150
Net gains (losses) arising during the period, (taxes) benefits	(41,129)	27,703	(3,753)
Amortization of net loss, prior service gain and transition obligation included in net periodic benefit cost, tax benefits	(10,041)	(9,267)	(14,344)
Reclassification adjustment for impact of D&Os of the PUC included in regulatory assets, (taxes) benefits	$ 49,523	$ (18,206)	$ 16,011