Bank segment - Reconciliation of Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Income Statements, Captions [Line Items]
Total revenues	$ 685,277	$ 632,281	$ 1,331,151	$ 1,223,843
Less: Retirement defined benefits expense—other than service costs	(1,564)	(1,906)	(3,397)	(3,782)
Total expenses	606,478	554,479	1,180,463	1,076,303
Operating income	78,799	77,802	150,688	147,540
Income before income taxes	59,582	59,626	112,858	111,208
American Savings Bank (ASB)
Condensed Income Statements, Captions [Line Items]
Interest and dividend income	63,261	59,080	125,263	116,802
Total noninterest income	13,843	16,249	27,260	31,383
Total revenues	77,104	75,329	152,523	148,185
Total interest expense	3,677	3,135	7,130	6,054
Provision for loan losses	2,763	2,834	6,304	6,741
Noninterest expense	44,150	44,564	88,075	86,434
Less: Retirement defined benefits expense—other than service costs	403	201	790	396
Total expenses	50,187	50,332	100,719	98,833
Operating income	26,917	24,997	51,804	49,352
Income before income taxes	$ 26,514	$ 24,796	$ 51,014	$ 48,956