Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Less: Treasury Common Stock [Member]	Less: Treasury Preferred Stock [Member]	Less: Unearned Employee Stock Ownership Plan Shares
Balance at Mar. 31, 2017	$ 2,619,744	$ 10,497	$ 452,172	$ (51,236)	$ 2,892,893	$ (525,653)	$ (151,997)	$ (6,932)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	447		447
Release of unearned ESOP shares	8,055							8,055
Purchase of ESOP shares	(6,764)							(6,764)
Foreign currency translation	19,240			19,240
Unrealized net gain (loss) on investments, net of tax	19,820			19,820
Change in fair value of cash flow hedges, net of tax	2,265			2,265
Net earnings	779,740				779,740
Common stock dividends	(29,380)				(29,380)
Net activity	793,423		447	41,325	750,360			1,291
Balance at Dec. 31, 2017	3,413,167	10,497	452,619	(9,911)	3,643,253	(525,653)	(151,997)	(5,641)
Balance at Sep. 30, 2017	2,892,422	10,497	452,474	(11,534)	3,124,152	(525,653)	(151,997)	(5,517)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	145		145
Release of unearned ESOP shares	2,680							2,680
Purchase of ESOP shares	(2,804)							(2,804)
Foreign currency translation	(1,794)			(1,794)
Unrealized net gain (loss) on investments, net of tax	2,852			2,852
Change in fair value of cash flow hedges, net of tax	565			565
Net earnings	528,894				528,894
Common stock dividends	(9,793)				(9,793)
Net activity	520,745		145	1,623	519,101			(124)
Balance at Dec. 31, 2017	3,413,167	10,497	452,619	(9,911)	3,643,253	(525,653)	(151,997)	(5,641)
Balance at Mar. 31, 2018	3,408,708	10,497	452,746	(4,623)	3,635,561	(525,653)	(151,997)	(7,535)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	370		370
Release of unearned ESOP shares	8,083							8,083
Purchase of ESOP shares	(203)							(5,403)
Foreign currency translation	(5,081)			(5,081)
Unrealized net gain (loss) on investments, net of tax	(55,830)			(55,830)
Change in fair value of cash flow hedges, net of tax	551			551
Adjustment for adoption of ASU 2018-02				(9,724)	9,724
Net earnings	370,026				370,026
Common stock dividends	(29,384)				(29,384)
Net activity	283,332		370	(70,084)	350,366			2,680
Balance at Dec. 31, 2018	3,692,040	10,497	453,116	(74,707)	3,985,927	(525,653)	(151,997)	(5,143)
Balance at Sep. 30, 2018	3,633,167	10,497	453,006	(62,238)	3,917,087	(525,653)	(151,997)	(7,535)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	110		110
Release of unearned ESOP shares	2,695							2,695
Purchase of ESOP shares	(303)							(303)
Foreign currency translation	(4,101)			(4,101)
Unrealized net gain (loss) on investments, net of tax	1,443			(8,281)
Change in fair value of cash flow hedges, net of tax	(87)			(87)
Adjustment for adoption of ASU 2018-02
Net earnings	78,635				78,635
Common stock dividends	(9,795)				(9,795)
Net activity	58,873		110	(12,469)	68,840			2,392
Balance at Dec. 31, 2018	$ 3,692,040	$ 10,497	$ 453,116	$ (74,707)	$ 3,985,927	$ (525,653)	$ (151,997)	$ (5,143)