Stockholders' Equity and EPS - Reclassification of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification adjustment for (gains) losses on available-for-sale securities included in net income									$ 420,752	$ 134,670	$ 88,094
Related tax expense (benefit)	$ (148,096)	$ (162,265)	$ (87,869)	$ (49,357)	$ (94,061)	$ (105,075)	$ (119,114)	$ (107,435)	(447,587)	(425,685)	(351,561)
Net interest income	591,480	527,740	512,927	524,137	533,668	520,644	529,403	512,886	2,156,284	2,096,601	1,893,988
Total reclassification adjustment for (gains) losses included in net income, net of tax	$ 388,316	$ 441,713	$ 228,935	$ 132,253	$ 262,856	$ 267,281	$ 317,987	$ 288,732	1,191,217	1,136,856	973,840
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total reclassification adjustment for (gains) losses included in net income, net of tax									(80,448)	6,687	535
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification adjustment for (gains) losses on available-for-sale securities included in net income									(61,165)	3,905	740
Related tax expense (benefit)									16,953	(1,087)	(205)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Related tax expense (benefit)									13,692	(1,489)	0
Net interest income									$ (49,928)	$ 5,358	$ 0