Note 4 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment securities available for sale, amortized cost	$ 1,267,731	$ 1,336,345
Investment securities available for sale, gross unrealized gains	1,778	13,965
Investment securities available for sale, gross unrealized losses	27,000	16,684
Investment securities available for sale, Estimated fair value	1,242,509	1,333,626
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	1,242,509	1,333,626
US Treasury Securities [Member]
Investment securities available for sale, amortized cost	124,801	249,877
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	50	357
Investment securities available for sale, Estimated fair value	124,751	249,520
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	124,751	249,520
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	6,066	9,047
Investment securities available for sale, gross unrealized gains		11
Investment securities available for sale, gross unrealized losses	195	70
Investment securities available for sale, Estimated fair value	5,871	8,988
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	5,871	8,988
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost	400,000	400,000
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	11,638	9,664
Investment securities available for sale, Estimated fair value	388,362	390,336
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	388,362	390,336
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	670,874	577,987
Investment securities available for sale, gross unrealized gains	960	241
Investment securities available for sale, gross unrealized losses	15,089	6,259
Investment securities available for sale, Estimated fair value	656,745	571,969
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	656,745	571,969
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale, amortized cost		1,944
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses		30
Investment securities available for sale, Estimated fair value		1,914
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)		1,914
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	1,005	1,533
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	28	17
Investment securities available for sale, Estimated fair value	977	1,516
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	977	1,516
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	64,985	80,007
Investment securities available for sale, gross unrealized gains	818	1,291
Investment securities available for sale, gross unrealized losses		17
Investment securities available for sale, Estimated fair value	65,803	81,281
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)	$ 65,803	81,281
Mutual Funds [Member]
Investment securities available for sale, amortized cost		6,500
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses		270
Investment securities available for sale, Estimated fair value		6,230
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)		6,230
Preferred Stock of Government Sponsored Entities [Member]
Investment securities available for sale, amortized cost		5,842
Investment securities available for sale, gross unrealized gains		4,260
Investment securities available for sale, gross unrealized losses
Investment securities available for sale, Estimated fair value		10,102
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)		10,102
Other Equity Securities [Member]
Investment securities available for sale, amortized cost		3,608
Investment securities available for sale, gross unrealized gains		8,162
Investment securities available for sale, gross unrealized losses
Investment securities available for sale, Estimated fair value		11,770
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017)		$ 11,770