Securities - Summary of Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Available-for-Sale Debt Securities:
Due within one year	$ 211,500
Due from one to five years	670,442
Due from five to ten years	1,128,312
Due after ten years	3,223,490
Total debt securities available for sale	5,233,744
Mortgage-related Securities
Available-for-Sale Debt Securities:
Due within one year	90,869
Due from one to five years	458,644
Due from five to ten years	148,256
Due after ten years	1,846,645
Total debt securities available for sale	$ 2,544,414
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	3.46%
Due from one to five years, Average Yield	3.11%
Due from five to ten years, Average Yield	2.74%
Due after ten years, Average Yield	2.51%
Total securities available for sale, Average Yield	2.67%
U.S. Government and GSE obligations
Available-for-Sale Debt Securities:
Due within one year	$ 68,935
Due from one to five years	25,924
Due from five to ten years	188,055
Due after ten years	770,936
Total debt securities available for sale	$ 1,053,850
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	0.48%
Due from one to five years, Average Yield	3.39%
Due from five to ten years, Average Yield	2.43%
Due after ten years, Average Yield	1.63%
Total securities available for sale, Average Yield	1.74%
State, County, and Municipal
Available-for-Sale Debt Securities:
Due within one year	$ 150
Due from five to ten years	20,274
Due after ten years	5,880
Total debt securities available for sale	$ 26,304
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	6.66%
Due from five to ten years, Average Yield	3.50%
Due after ten years, Average Yield	3.33%
Total securities available for sale, Average Yield	3.48%
Other Debt Securities
Available-for-Sale Debt Securities:
Due within one year	$ 49,593
Due from one to five years	146,882
Due from five to ten years	761,267
Due after ten years	549,646
Total debt securities available for sale	$ 1,507,388
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	3.65%
Due from one to five years, Average Yield	2.85%
Due from five to ten years, Average Yield	2.19%
Due after ten years, Average Yield	1.39%
Total securities available for sale, Average Yield	2.01%