Bank segment - Comprehensive income data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Condensed Statement of Income Captions [Line Items]
Net income	$ 32,339	$ 46,260
Net unrealized gains (losses) on securities:
Net unrealized gains losses on available-for-sale investment securities arising during the period, net of tax benefits of $2,278 and $1,664 for the respective periods	3,451	2,520
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil and $1,132 for the respective periods	0	(1,715)
Retirement benefit plans:
Less: amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits of $259 and $144 for the respective periods	5,459	2,813
Other comprehensive income, net of taxes	4,058	1,167
Comprehensive income	35,924	46,954
Net unrealized gains (losses) on securities arising during the period, taxes	(2,278)	(1,664)
Less: reclassification adjustment for net realized gains included in net income, taxes	0	(1,132)
Less: amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits	(3,486)	(1,796)
American Savings Bank (ASB)
Condensed Statement of Income Captions [Line Items]
Net income	13,475	14,399
Net unrealized gains (losses) on securities:
Net unrealized gains losses on available-for-sale investment securities arising during the period, net of tax benefits of $2,278 and $1,664 for the respective periods	3,451	2,520
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil and $1,132 for the respective periods	0	(1,715)
Retirement benefit plans:
Less: amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits of $259 and $144 for the respective periods	392	219
Other comprehensive income, net of taxes	3,843	1,024
Comprehensive income	17,318	15,423
Net unrealized gains (losses) on securities arising during the period, taxes	2,278	1,664
Less: reclassification adjustment for net realized gains included in net income, taxes	0	1,132
Less: amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits	$ 259	$ (144)