Condensed Statements of Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Interest income	$ 1,805,160	$ 1,689,673	$ 1,582,239
Dividends received from subsidiaries	(380,000)
Other income	18,845	35,535	45,662
Operating expenses	511,218	546,628	641,218
Income tax benefit	(128,264)	(135,252)	(202,014)
Net income	395,043	422,417	466,201
Parent Company
Condensed Financial Statements, Captions [Line Items]
Interest income	668	500	943
Dividends received from subsidiaries	380,000	380,000	336,000
Other income	716	793	1,700
Gross income	381,384	381,293	338,643
Operating expenses	49,926	59,372	54,333
Income before income tax benefit and equity in underdistributed earnings of subsidiaries	331,458	321,921	284,310
Income tax benefit	13,669	16,616	19,575
Income before equity in underdistributed earnings of subsidiaries	345,127	338,537	303,885
Equity in underdistributed earnings of subsidiaries	49,916	83,880	162,316
Net income	$ 395,043	$ 422,417	$ 466,201