Note 3 - Investment Securities - Gross Unrealized Losses, Investment Securities Portfolio (Details) $ in Thousands	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Available for sale securities, less than 12 months, number of investment positions	16	16
Available for sale securities, less than 12 months, fair value	$ 226,864	$ 96,910
Available for sale securities, less than 12 months, unrealized losses	$ (1,015)	$ (314)
Available for sale securities, 12 months or longer, number of investment positions	8	70
Available for sale securities, 12 months or longer, fair value	$ 22,915	$ 462,348
Available for sale securities, 12 months or longer, unrealized losses	$ (400)	$ (6,557)
Available for sale securities, total, number of investment positions	24	86
Available for sale securities, total, fair value	$ 249,779	$ 559,258
Available for sale securities, total, unrealized losses	$ (1,415)	$ (6,871)
Held to Maturity securities, less than 12 months, number of investment positions	4	6
Held to Maturity securities, less than 12 months, fair value	$ 686	$ 12,098
Held to Maturity securities, less than 12 months, unrecognized losses	$ 0	$ (87)
Held to Maturity securities, 12 months or longer, number of investment positions	4	55
Held to Maturity securities, 12 months or longer, fair value	$ 1,883	$ 277,454
Held to Maturity securities, 12 months or longer, unrecognized losses	$ (45)	$ (2,149)
Held to Maturity securities, total, number of investment positions	8	61
Held to Maturity securities, total, fair value	$ 2,569	$ 289,552
Held to Maturity securities, total, unrecognized losses	$ (45)	$ (2,236)
Agency Residential MBS [Member]
Available for sale securities, less than 12 months, number of investment positions	0	6
Available for sale securities, less than 12 months, fair value	$ 0	$ 11,674
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ (100)
Available for sale securities, 12 months or longer, number of investment positions	2	47
Available for sale securities, 12 months or longer, fair value	$ 337	$ 347,384
Available for sale securities, 12 months or longer, unrealized losses	$ (1)	$ (5,738)
Available for sale securities, total, number of investment positions	2	53
Available for sale securities, total, fair value	$ 337	$ 359,058
Available for sale securities, total, unrealized losses	$ (1)	$ (5,838)
Held to Maturity securities, less than 12 months, number of investment positions	3	6
Held to Maturity securities, less than 12 months, fair value	$ 179	$ 12,098
Held to Maturity securities, less than 12 months, unrecognized losses	$ 0	$ (87)
Held to Maturity securities, 12 months or longer, number of investment positions	4	54
Held to Maturity securities, 12 months or longer, fair value	$ 1,883	$ 277,203
Held to Maturity securities, 12 months or longer, unrecognized losses	$ (45)	$ (2,148)
Held to Maturity securities, total, number of investment positions	7	60
Held to Maturity securities, total, fair value	$ 2,062	$ 289,301
Held to Maturity securities, total, unrecognized losses	$ (45)	$ (2,235)
US Government-sponsored Enterprises Debt Securities [Member]
Available for sale securities, less than 12 months, number of investment positions		1
Available for sale securities, less than 12 months, fair value		$ 9,951
Available for sale securities, less than 12 months, unrealized losses		$ (49)
Available for sale securities, 12 months or longer, number of investment positions		3
Available for sale securities, 12 months or longer, fair value		$ 45,877
Available for sale securities, 12 months or longer, unrealized losses		$ (49)
Available for sale securities, total, number of investment positions		4
Available for sale securities, total, fair value		$ 55,828
Available for sale securities, total, unrealized losses		$ (98)
Agency Commercial MBS [Member]
Available for sale securities, less than 12 months, number of investment positions	1	1
Available for sale securities, less than 12 months, fair value	$ 3,629	$ 3,708
Available for sale securities, less than 12 months, unrealized losses	$ (9)	$ (3)
Available for sale securities, 12 months or longer, number of investment positions	0	0
Available for sale securities, 12 months or longer, fair value	$ 0	$ 0
Available for sale securities, 12 months or longer, unrealized losses	$ 0	$ 0
Available for sale securities, total, number of investment positions	1	1
Available for sale securities, total, fair value	$ 3,629	$ 3,708
Available for sale securities, total, unrealized losses	$ (9)	$ (3)
Non-agency Residential MBS [Member]
Held to Maturity securities, less than 12 months, number of investment positions	1
Held to Maturity securities, less than 12 months, fair value	$ 507
Held to Maturity securities, less than 12 months, unrecognized losses	$ 0
Held to Maturity securities, 12 months or longer, number of investment positions	0
Held to Maturity securities, 12 months or longer, fair value	$ 0
Held to Maturity securities, 12 months or longer, unrecognized losses	$ 0
Held to Maturity securities, total, number of investment positions	1
Held to Maturity securities, total, fair value	$ 507
Held to Maturity securities, total, unrecognized losses	$ 0
US States and Political Subdivisions Debt Securities [Member]
Available for sale securities, less than 12 months, number of investment positions	0	0
Available for sale securities, less than 12 months, fair value	$ 0	$ 0
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ 0
Available for sale securities, 12 months or longer, number of investment positions	2	7
Available for sale securities, 12 months or longer, fair value	$ 2,169	$ 4,163
Available for sale securities, 12 months or longer, unrealized losses	$ (38)	$ (44)
Available for sale securities, total, number of investment positions	2	7
Available for sale securities, total, fair value	$ 2,169	$ 4,163
Available for sale securities, total, unrealized losses	$ (38)	$ (44)
Held to Maturity securities, less than 12 months, number of investment positions		0
Held to Maturity securities, less than 12 months, fair value		$ 0
Held to Maturity securities, less than 12 months, unrecognized losses		$ 0
Held to Maturity securities, 12 months or longer, number of investment positions		1
Held to Maturity securities, 12 months or longer, fair value		$ 251
Held to Maturity securities, 12 months or longer, unrecognized losses		$ (1)
Held to Maturity securities, total, number of investment positions		1
Held to Maturity securities, total, fair value		$ 251
Held to Maturity securities, total, unrecognized losses		$ (1)
US Government Agencies Debt Securities [Member]
Available for sale securities, less than 12 months, number of investment positions	1	0
Available for sale securities, less than 12 months, fair value	$ 171	$ 0
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ 0
Available for sale securities, 12 months or longer, number of investment positions	0	2
Available for sale securities, 12 months or longer, fair value	$ 0	$ 544
Available for sale securities, 12 months or longer, unrealized losses	$ 0	$ (9)
Available for sale securities, total, number of investment positions	1	2
Available for sale securities, total, fair value	$ 171	$ 544
Available for sale securities, total, unrealized losses	$ 0	$ (9)
Debt Security, Corporate, US [Member]
Available for sale securities, less than 12 months, number of investment positions	5	8
Available for sale securities, less than 12 months, fair value	$ 44,582	$ 71,577
Available for sale securities, less than 12 months, unrealized losses	$ (749)	$ (162)
Available for sale securities, 12 months or longer, number of investment positions	4	11
Available for sale securities, 12 months or longer, fair value	$ 20,409	$ 64,380
Available for sale securities, 12 months or longer, unrealized losses	$ (361)	$ (717)
Available for sale securities, total, number of investment positions	9	19
Available for sale securities, total, fair value	$ 64,991	$ 135,957
Available for sale securities, total, unrealized losses	$ (1,110)	$ (879)
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Available for sale securities, less than 12 months, number of investment positions	6
Available for sale securities, less than 12 months, fair value	$ 149,670
Available for sale securities, less than 12 months, unrealized losses	$ (205)
Available for sale securities, 12 months or longer, number of investment positions	0
Available for sale securities, 12 months or longer, fair value	$ 0
Available for sale securities, 12 months or longer, unrealized losses	$ 0
Available for sale securities, total, number of investment positions	6
Available for sale securities, total, fair value	$ 149,670
Available for sale securities, total, unrealized losses	$ (205)
Collateralized Loan Obligations [Member]
Available for sale securities, less than 12 months, number of investment positions	3
Available for sale securities, less than 12 months, fair value	$ 28,812
Available for sale securities, less than 12 months, unrealized losses	$ (52)
Available for sale securities, 12 months or longer, number of investment positions	0
Available for sale securities, 12 months or longer, fair value	$ 0
Available for sale securities, 12 months or longer, unrealized losses	$ 0
Available for sale securities, total, number of investment positions	3
Available for sale securities, total, fair value	$ 28,812
Available for sale securities, total, unrealized losses	$ (52)