New Accounting Pronouncements - Additional Information (Details) - USD ($)	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Oct. 01, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities, at amortized cost	$ 827,316,000				$ 1,443,480,000
Available-for-sale, at amortized cost	2,022,414,000				1,458,071,000
Mortgage-backed securities available-for-sale, fair value			$ 390,669,000
Increase in other asssets	342,658,000				210,989,000
Decrease to retained earnings	1,990,509,000	$ 1,964,612,000			2,032,995,000	$ 2,012,734,000	$ 2,004,280,000	$ 1,996,908,000
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings					(21,945,000)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings	$ 1,403,124,000	$ 1,384,833,000			1,335,909,000	$ 1,300,116,000	$ 1,262,236,000	$ 1,188,971,000
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings				$ (21,945,000)	$ (21,945,000)
Accounting Standards Update 2019-04
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Mortgage-backed securities held-to-maturity, fair value			(390,669,000)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in other asssets				29,013,000
Increase in other liabilities				29,013,000
Accounting Standards Update 2016-02 | Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings				$ 0
Mortgage-backed securities | Accounting Standards Update 2019-04
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities, at amortized cost			(374,680,000)
Available-for-sale, at amortized cost			$ 374,680,000