Securities - Scheduled Contractual Maturities of AFS (Details) $ in Thousands	Mar. 31, 2025 USD ($)
Amortized cost
Within One Year	$ 500,038
After One Year through Five Years	787,650
After Five Years through Ten Years	772,987
After Ten Years	10,901,794
Amortized Cost	12,962,469
Fair value
Within One Year	499,502
After One Year through Five Years	753,139
After Five Years through Ten Years	704,467
After Ten Years	10,427,804
Total	$ 12,384,912
Weighted-average yield
Within One Year	3.80%
After One Year through Five Years	1.37%
After Five Years through Ten Years	3.23%
After Ten Years	4.70%
Total	4.37%
U.S. Treasury securities
Amortized cost
Within One Year	$ 369,423
After One Year through Five Years	591,868
After Five Years through Ten Years	0
After Ten Years	0
Amortized Cost	961,291
Fair value
Within One Year	368,978
After One Year through Five Years	562,003
After Five Years through Ten Years	0
After Ten Years	0
Total	$ 930,981
Weighted-average yield
Within One Year	4.10%
After One Year through Five Years	1.06%
After Five Years through Ten Years	0.00%
After Ten Years	0.00%
Total	2.23%
U.S. government agency and U.S. government-sponsored enterprise debt securities
Amortized cost
Within One Year	$ 16,596
After One Year through Five Years	26,929
After Five Years through Ten Years	201,232
After Ten Years	61,671
Amortized Cost	306,428
Fair value
Within One Year	16,367
After One Year through Five Years	25,806
After Five Years through Ten Years	174,409
After Ten Years	51,824
Total	$ 268,406
Weighted-average yield
Within One Year	0.93%
After One Year through Five Years	1.36%
After Five Years through Ten Years	2.00%
After Ten Years	2.00%
Total	1.89%
U.S. government agency and U.S. government-sponsored enterprise mortgage-backed securities
Amortized cost
Within One Year	$ 5,057
After One Year through Five Years	74,745
After Five Years through Ten Years	115,210
After Ten Years	9,465,345
Amortized Cost	9,660,357
Fair value
Within One Year	5,018
After One Year through Five Years	71,910
After Five Years through Ten Years	105,670
After Ten Years	9,241,514
Total	$ 9,424,112
Weighted-average yield
Within One Year	3.56%
After One Year through Five Years	2.77%
After Five Years through Ten Years	2.86%
After Ten Years	5.06%
Total	5.02%
Municipal securities
Amortized cost
Within One Year	$ 6,637
After One Year through Five Years	25,434
After Five Years through Ten Years	8,111
After Ten Years	246,192
Amortized Cost	286,374
Fair value
Within One Year	6,551
After One Year through Five Years	24,365
After Five Years through Ten Years	7,591
After Ten Years	207,347
Total	$ 245,854
Weighted-average yield
Within One Year	2.09%
After One Year through Five Years	2.17%
After Five Years through Ten Years	3.38%
After Ten Years	2.23%
Total	2.25%
Non-agency mortgage-backed securities
Amortized cost
Within One Year	$ 20,756
After One Year through Five Years	5,784
After Five Years through Ten Years	4,434
After Ten Years	740,349
Amortized Cost	771,323
Fair value
Within One Year	20,558
After One Year through Five Years	5,641
After Five Years through Ten Years	4,433
After Ten Years	636,036
Total	$ 666,668
Weighted-average yield
Within One Year	6.10%
After One Year through Five Years	3.10%
After Five Years through Ten Years	5.51%
After Ten Years	2.38%
Total	2.51%
Corporate debt securities
Amortized cost
Within One Year	$ 0
After One Year through Five Years	0
After Five Years through Ten Years	349,500
After Ten Years	304,000
Amortized Cost	653,500
Fair value
Within One Year	0
After One Year through Five Years	0
After Five Years through Ten Years	318,192
After Ten Years	218,350
Total	$ 536,542
Weighted-average yield
Within One Year	0.00%
After One Year through Five Years	0.00%
After Five Years through Ten Years	3.50%
After Ten Years	1.97%
Total	2.79%
Foreign government bonds
Amortized cost
Within One Year	$ 81,569
After One Year through Five Years	62,890
After Five Years through Ten Years	50,000
After Ten Years	50,000
Amortized Cost	244,459
Fair value
Within One Year	82,030
After One Year through Five Years	63,414
After Five Years through Ten Years	49,749
After Ten Years	38,998
Total	$ 234,191
Weighted-average yield
Within One Year	2.56%
After One Year through Five Years	2.16%
After Five Years through Ten Years	4.74%
After Ten Years	1.50%
Total	2.69%
Asset-backed securities
Amortized cost
Within One Year	$ 0
After One Year through Five Years	0
After Five Years through Ten Years	0
After Ten Years	34,237
Amortized Cost	34,237
Fair value
Within One Year	0
After One Year through Five Years	0
After Five Years through Ten Years	0
After Ten Years	33,735
Total	$ 33,735
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	5.05%
Total	5.05%
CLOs
Amortized cost
Within One Year	$ 0
After One Year through Five Years	0
After Five Years through Ten Years	44,500
After Ten Years	0
Amortized Cost	44,500
Fair value
Within One Year	0
After One Year through Five Years	0
After Five Years through Ten Years	44,423
After Ten Years	0
Total	$ 44,423
Weighted-average yield
Within One Year	0.00%
After One Year through Five Years	0.00%
After Five Years through Ten Years	5.75%
After Ten Years	0.00%
Total	5.75%