Stockholders' Equity and EPS - Reclassification out of AOCI (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains on investment securities, net	$ 29,028,000	$ 9,058,000
Related tax benefit	(107,435,000)	(73,966,000)
Net interest income	512,886,000	419,863,000
Total reclassification adjustment for losses included in net income, net of tax	288,732,000	194,961,000
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassification adjustment for losses included in net income, net of tax	2,622,000	0
Accumulated Net Investment Gain (Loss) Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains on investment securities, net	3,630,000	0
Related tax benefit	(1,010,000)	0
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Related tax benefit	(1,000)	0
Net interest income	$ 3,000	$ 0