Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2023		Dec. 31, 2022
Investment securities available for sale	$ 3,906,233		$ 4,331,743
Agency Residential MBS [Member]
Investment securities available for sale	239,728		286,048
US Government Agencies Debt Securities [Member]
Investment securities available for sale	279,364		290,853
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale	69,639		82,004
Corporate Debt Securities [Member]
Investment securities available for sale	1,814,424		2,099,955
Fair Value, Recurring [Member]
Investment securities available for sale	3,906,233		4,331,743
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	3,906,233		4,331,743
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	239,728		286,048
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	239,728		286,048
Fair Value, Recurring [Member] | Agency Residential MBS [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale	0		0	[2]
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member]
Investment securities available for sale	279,364		290,853
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	279,364		290,853
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale	0		0	[2]
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale	69,639		82,004
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	69,639		82,004
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] | Corporate Debt Securities [Member]
Investment securities available for sale	1,814,424		2,099,955
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale	0		0
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale	1,814,424		2,099,955
Fair Value, Recurring [Member] | Corporate Debt Securities [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	1,503,078		1,572,883
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	1,503,078		1,572,883
Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale	$ 0	[1]	$ 0	[2]

[1] There were no transfers in to or out of level 3 during the six months ended June 30, 2023.

[2] There were no transfers in to or out of level 3 during the year ended December 31, 2022.