Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands		Mar. 31, 2021		Dec. 31, 2020
Investment securities available for sale		$ 3,990,570		$ 4,063,185
Agency Residential MBS [Member]
Investment securities available for sale		577,490		652,952
US Government Agencies Debt Securities [Member]
Investment securities available for sale		145		154
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale		106,117		111,010
Debt Security, Corporate, US [Member]
Investment securities available for sale		2,149,366		2,117,978
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investment securities available for sale				24,990
Fair Value, Recurring [Member]
Investment securities available for sale		3,990,570		4,063,185
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale		0		0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale		3,990,570		4,063,185
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale		0	[1]	0	[2]
Fair Value, Recurring [Member] | Agency Residential MBS [Member]
Investment securities available for sale		577,490		652,952
Fair Value, Recurring [Member] | Agency Residential MBS [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale		0		0
Fair Value, Recurring [Member] | Agency Residential MBS [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale		577,490		652,952
Fair Value, Recurring [Member] | Agency Residential MBS [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale		0	[1]	0	[2]
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member]
Investment securities available for sale		145		154
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale		0		0
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale		145		154
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale		0	[1]	0	[2]
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale		106,117		111,010
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale		0		0
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale		106,117		111,010
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale		0	[1]	0	[2]
Fair Value, Recurring [Member] | Debt Security, Corporate, US [Member]
Investment securities available for sale		2,149,366		2,117,978
Fair Value, Recurring [Member] | Debt Security, Corporate, US [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale		0		0
Fair Value, Recurring [Member] | Debt Security, Corporate, US [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale		2,149,366		2,117,978
Fair Value, Recurring [Member] | Debt Security, Corporate, US [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale		0	[1]	0	[2]
Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member]
Investment securities available for sale		1,157,452		1,156,101
Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale		0		0
Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale		1,157,452		1,156,101
Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale		$ 0	[1]	0	[2]
Fair Value, Recurring [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investment securities available for sale				24,990
Fair Value, Recurring [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale				0
Fair Value, Recurring [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale				24,990
Fair Value, Recurring [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale	[2]			$ 0

[1] There were no transfers in to or out of level 3 during the three months ended March 31, 2021.

[2] There were no transfers in to or out of level 3 during the year ended December 31, 2020.