Bank segment - Comprehensive income data (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Condensed Statement of Income Captions [Line Items]
Net income	$ 51,144	$ 48,279	$ 118,974	$ 136,293
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on available-for-sale investment securities arising during the period, net of (taxes) benefits of $(2,543), $1,094, $(2,382) and $(2,249) for the respective periods	3,851	(1,657)	3,608	3,406
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil, nil, nil and $1,132 for the respective periods	0	0	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 $249, $138, $763 and $424 for the respective periods	5,611	2,829	16,850	8,515
Other comprehensive income (loss), net of taxes	4,430	(1,311)	5,361	2,756
Comprehensive income	55,103	46,497	122,918	137,632
Net unrealized gains (losses) on securities arising during the period, taxes	(2,543)	1,094	(2,382)	(2,249)
Less: reclassification adjustment for net realized gains included in net income, taxes	0	0	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	3,583	1,900	10,760	5,438
American Savings Bank (ASB)
Condensed Statement of Income Captions [Line Items]
Net income	13,451	13,253	39,777	39,188
Net unrealized gains (losses) on securities:
Net unrealized gains (losses) on available-for-sale investment securities arising during the period, net of (taxes) benefits of $(2,543), $1,094, $(2,382) and $(2,249) for the respective periods	3,851	(1,657)	3,608	3,406
Less: reclassification adjustment for net realized gains included in net income, net of taxes of nil, nil, nil and $1,132 for the respective periods	0	0	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 $249, $138, $763 and $424 for the respective periods	376	208	1,155	642
Other comprehensive income (loss), net of taxes	4,227	(1,449)	4,763	2,333
Comprehensive income	17,678	11,804	44,540	41,521
Net unrealized gains (losses) on securities arising during the period, taxes	2,543	(1,094)	2,382	2,249
Less: reclassification adjustment for net realized gains included in net income, taxes	0	0	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	$ 249	$ 138	$ 763	$ 424