Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Total accumulated other comprehensive income	$ 1,183,261	$ 1,236,081	$ 1,268,748	$ 989,273	$ 1,014,233	$ 1,057,181
Net Unrealized Loss on Securities Available for Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income, before tax	(7,992)		(4,321)
Tax effect	2,313		1,159
Total accumulated other comprehensive income	(5,679)		(3,162)
Net Unrealized Loss on Securities Transferred from Available for Sale to Held to Maturity [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income, before tax	(720)		(887)
Tax effect	212		249
Total accumulated other comprehensive income	(508)		(638)
Fair Value Adjustments on Derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income, before tax	22,109		31,881
Tax effect	(6,521)		(8,961)
Total accumulated other comprehensive income	15,588		22,920
Benefit Plan Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income, before tax	(850)		(813)
Tax effect	251		228
Total accumulated other comprehensive income	(599)		(585)
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Total accumulated other comprehensive income	$ 8,802	$ 18,818	$ 18,535	$ 6,412	$ 3,140	$ 1,044