Securities - Scheduled Contractual Maturities of HTM Debt Securities (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Amortized cost
Within One Year	$ 0
After One Year through Five Years	528,169
After Five Years through Ten Years	423,862
After Ten Years	2,012,204
Total	2,964,235
Fair value
Within One Year	0
After One Year through Five Years	470,618
After Five Years through Ten Years	341,533
After Ten Years	1,495,897
Total	$ 2,308,048
Weighted Average Yield
Within One Year	0.00%
After One Year through Five Years	1.05%
After Five Years through Ten Years	1.84%
After Ten Years	1.78%
Total	1.66%
U.S. Treasury securities
Amortized cost
Within One Year	$ 0
After One Year through Five Years	528,169
After Five Years through Ten Years	0
After Ten Years	0
Total	528,169
Fair value
Within One Year	0
After One Year through Five Years	470,618
After Five Years through Ten Years	0
After Ten Years	0
Total	$ 470,618
Weighted Average Yield
Within One Year	0.00%
After One Year through Five Years	1.05%
After Five Years through Ten Years	0.00%
After Ten Years	0.00%
Total	1.05%
U.S. government-sponsored enterprises debt securities
Amortized cost
Within One Year	$ 0
After One Year through Five Years	0
After Five Years through Ten Years	328,870
After Ten Years	672,428
Total	1,001,298
Fair value
Within One Year	0
After One Year through Five Years	0
After Five Years through Ten Years	265,107
After Ten Years	487,184
Total	$ 752,291
Weighted Average Yield
Within One Year	0.00%
After One Year through Five Years	0.00%
After Five Years through Ten Years	1.92%
After Ten Years	1.89%
Total	1.90%
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities
Amortized cost
Within One Year	$ 0
After One Year through Five Years	0
After Five Years through Ten Years	94,992
After Ten Years	1,150,683
Total	1,245,675
Fair value
Within One Year	0
After One Year through Five Years	0
After Five Years through Ten Years	76,426
After Ten Years	873,138
Total	$ 949,564
Weighted Average Yield
Within One Year	0.00%
After One Year through Five Years	0.00%
After Five Years through Ten Years	1.56%
After Ten Years	1.68%
Total	1.67%
Municipal securities
Amortized cost
Within One Year	$ 0
After One Year through Five Years	0
After Five Years through Ten Years	0
After Ten Years	189,093
Total	189,093
Fair value
Within One Year	0
After One Year through Five Years	0
After Five Years through Ten Years	0
After Ten Years	135,575
Total	$ 135,575
Weighted Average Yield
Within One Year	0.00%
After One Year through Five Years	0.00%
After Five Years through Ten Years	0.00%
After Ten Years	1.99%
Total	1.99%