Note 21 - Regulatory Matters - Capital and Leverage Ratios (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Common equity Tier 1 capital ( to risk-weighted assets)		$ 1,572,025	$ 1,459,351
Common equity Tier 1 capital ( to risk-weighted assets)		12.19%	12.84%
Common equity Tier 1 capital minimum requirement		$ 580,552	$ 511,590
Common equity Tier 1 capital minimum requirement		4.50%	4.50%
Excess Common equtiy Tier 1 capital		$ 991,473	$ 947,761
Excess Common equtiy Tier 1 capital		7.69%	8.34%
Tier 1 capital (to risk-weighted assets)		$ 1,572,025	$ 1,574,806
Tier 1 capital (to risk-weighted assets)		12.19%	13.85%
Tier 1 capital minimum requirement		$ 774,070	$ 682,120
Tier 1 capital minimum requirement		6.00%	6.00%
Excess Tier 1 capital		$ 797,955	$ 892,686
Excess Tier 1 capital		6.19%	7.85%
Total capital (to risk-weighted assets)		$ 1,820,860	$ 1,702,144
Total capital (to risk-weighted assets)		14.11%	14.97%
Total capital minimum requirement		$ 1,032,093	$ 909,493
Total capital minimum requirement		8.00%	8.00%
Excess Total capital		$ 788,767	$ 792,651
Excess Total capital		6.11%	6.97%
Tier 1 capital (to average assets) – Leverage ratio		$ 1,572,025	$ 1,574,806
Tier 1 capital (to average assets) – Leverage ratio		10.35%	11.57%
Minimum leverage requirement		$ 607,349	$ 544,614
Minimum leverage requirement		4.00%	4.00%
Excess Tier One Leverage Capital		$ 964,676	$ 1,030,192
Excess Tier One Leverage Capital		6.35%	7.57%
Risk-weighted assets		$ 12,901,161	$ 11,368,663
Total average assets (1)	[1]	15,183,720	13,615,348
Bank [Member]
Common equity Tier 1 capital ( to risk-weighted assets)		$ 1,734,719	$ 1,515,096
Common equity Tier 1 capital ( to risk-weighted assets)		13.46%	13.35%
Common equity Tier 1 capital minimum requirement		$ 579,921	$ 510,582
Common equity Tier 1 capital minimum requirement		4.50%	4.50%
Excess Common equtiy Tier 1 capital		$ 1,154,798	$ 1,004,514
Excess Common equtiy Tier 1 capital		8.96%	8.85%
Tier 1 capital (to risk-weighted assets)		$ 1,734,719	$ 1,515,096
Tier 1 capital (to risk-weighted assets)		13.46%	13.35%
Tier 1 capital minimum requirement		$ 773,229	$ 680,776
Tier 1 capital minimum requirement		6.00%	6.00%
Excess Tier 1 capital		$ 961,490	$ 834,320
Excess Tier 1 capital		7.46%	7.35%
Total capital (to risk-weighted assets)		$ 1,862,806	$ 1,637,286
Total capital (to risk-weighted assets)		14.45%	14.43%
Total capital minimum requirement		$ 1,030,971	$ 907,701
Total capital minimum requirement		8.00%	8.00%
Excess Total capital		$ 831,835	$ 729,585
Excess Total capital		6.45%	6.43%
Tier 1 capital (to average assets) – Leverage ratio		$ 1,734,719	$ 1,515,096
Tier 1 capital (to average assets) – Leverage ratio		11.82%	11.16%
Minimum leverage requirement		$ 586,959	$ 543,059
Minimum leverage requirement		4.00%	4.00%
Excess Tier One Leverage Capital		$ 1,147,760	$ 972,037
Excess Tier One Leverage Capital		7.82%	7.16%
Risk-weighted assets		$ 12,887,142	$ 11,346,260
Total average assets (1)	[1]	$ 14,673,981	$ 13,576,477

[1] The quarterly total average assets reflect all debt securities at amortized cost, equity security with readily determinable fair values at the lower of cost or fair value, and equity securities without readily determinable fair values at historical cost.