Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019		Dec. 31, 2018		Jan. 01, 2018
Equity securities	$ 1,797		$ 1,747		$ 1,800
Investment securities available for sale	2,775,899		2,654,670
US Treasury Securities [Member]
Investment securities available for sale	44,573		139,574
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale	167,138		164,018
Agency Residential MBS [Member]
Investment securities available for sale	986,946		853,871
Non-agency Residential MBS [Member]
Investment securities available for sale	112		114
Agency Commercial MBS [Member]
Investment securities available for sale	5,621		1,842
US Government Agencies Debt Securities [Member]
Investment securities available for sale	808		1,119
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale	175,654		179,091
Debt Security, Corporate, US [Member]
Investment securities available for sale	1,395,047		1,315,041
Fair Value, Recurring [Member]
Equity securities	1,797		1,747
Investment securities available for sale	2,775,899		2,654,670
Total	2,777,696		2,656,417
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Equity securities	0		0
Investment securities available for sale	44,573		139,574
Total	44,573		139,574
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Equity securities	1,797		1,747
Investment securities available for sale	2,731,326		2,515,096
Total	2,733,123		2,516,843
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Equity securities	0	[1]	0	[2]
Investment securities available for sale	0	[1]	0	[2]
Total	0	[1]	0	[2]
Fair Value, Recurring [Member] | Mutual Fund [Member]
Equity securities	1,797		1,747
Fair Value, Recurring [Member] | Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Equity securities	0		0
Fair Value, Recurring [Member] | Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member]
Equity securities	1,797		1,747
Fair Value, Recurring [Member] | Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member]
Equity securities	0	[1]	0	[2]
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Investment securities available for sale	44,573		139,574
Fair Value, Recurring [Member] | US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale	44,573		139,574
Fair Value, Recurring [Member] | US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale	0		0
Fair Value, Recurring [Member] | US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale	0	[1]	0	[2]
Fair Value, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale	167,138		164,018
Fair Value, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale	0		0
Fair Value, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale	167,138		164,018
Fair Value, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [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	986,946		853,871
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	986,946		853,871
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] | Non-agency Residential MBS [Member]
Investment securities available for sale	112		114
Fair Value, Recurring [Member] | Non-agency Residential MBS [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale	0		0
Fair Value, Recurring [Member] | Non-agency Residential MBS [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale	112		114
Fair Value, Recurring [Member] | Non-agency Residential MBS [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale	0	[1]	0	[2]
Fair Value, Recurring [Member] | Agency Commercial MBS [Member]
Investment securities available for sale	5,621		1,842
Fair Value, Recurring [Member] | Agency Commercial MBS [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale	0		0
Fair Value, Recurring [Member] | Agency Commercial MBS [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale	5,621		1,842
Fair Value, Recurring [Member] | Agency Commercial 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	808		1,119
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	808		1,119
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	175,654		179,091
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	175,654		179,091
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	1,395,047		1,315,041
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	1,395,047		1,315,041
Fair Value, Recurring [Member] | Debt Security, Corporate, US [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, 2019.

[2] There were no transfers in to or out of level 3 during the year ended December 31, 2018.