Bank segment - Comprehensive income data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Statement of Income Captions [Line Items]
Net income	$ 44,601	$ 35,491	$ 77,426	$ 67,830
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 ($1,925), $2,439, ($6,830) and $161 for the respective periods	2,916	(3,694)	10,344	(243)
Less: reclassification adjustment for net realized gains included in net income, net of taxes of $238, nil, $238 and nil for the respective periods	(360)	0	(360)	0
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 $140, $255, $277 and $514 for the respective periods	3,698	5,780	7,236	11,239
Other comprehensive income (loss), net of taxes	2,108	(3,127)	10,908	931
Comprehensive income	46,236	31,891	87,388	67,815
Net unrealized gains (losses) on securities arising during the period, taxes	1,925	(2,439)	6,830	(161)
Reclassification adjustment for net realized gain included in net income, taxes	238	0	238	0
Less: amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits	(2,362)	(3,691)	(4,619)	(7,177)
American Savings Bank (ASB)
Condensed Statement of Income Captions [Line Items]
Net income	13,285	12,851	25,958	26,326
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 ($1,925), $2,439, ($6,830) and $161 for the respective periods	2,915	(3,694)	10,344	(243)
Less: reclassification adjustment for net realized gains included in net income, net of taxes of $238, nil, $238 and nil for the respective periods	(360)	0	(360)	0
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 $140, $255, $277 and $514 for the respective periods	211	387	419	779
Other comprehensive income (loss), net of taxes	2,766	(3,307)	10,403	536
Comprehensive income	16,051	9,544	36,361	26,862
Net unrealized gains (losses) on securities arising during the period, taxes	(1,925)	2,439	(6,830)	161
Reclassification adjustment for net realized gain included in net income, taxes	238	0	238	0
Less: amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits	$ (140)	$ (255)	$ (277)	$ (514)