New Accounting Pronouncements (Details) - USD ($)	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities, at amortized cost	$ 920,255,000				$ 1,443,480,000
Available-for-sale, at amortized cost	1,667,467,000				1,458,071,000
Mortgage-backed securities available-for-sale, fair value		$ 390,669,000
Allowance for loan losses	139,501,000		$ 132,513,000		131,534,000	$ 133,086,000	$ 131,165,000	$ 129,257,000
Increase in other asssets	303,467,000				$ 210,989,000
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 2016-13 | Pro Forma
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan losses	$ 176,000,000
Allowance for credit losses, percentage of gross loans outstanding	1.31%
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
Cumulative effect adjustment 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