Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income	$ 25,940	$ 48,015	$ 176,042	$ 175,099	$ 161,109
Adjustments to reconcile net income to net cash provided by operating activities:
Reversal for credit losses			(2,500)	(15,650)	(11,400)
Provision for losses on other real estate owned			691	176	547
Deferred tax liability.			34,554	15,949	2,004
Depreciation and amortization			7,265	7,490	7,574
Net gains on sale and transfers of other real estate owned			(2,661)	(546)	(2,012)
Net gains on sale of loans				(285)	(786)
Proceeds from sale of loans			7,500	20,079	32,530
Originations of loans held for sale				(12,665)	(37,447)
Amortization on alternative energy partnerships, venture capital and other investments			4,572	28,897	25,058
Write-down on impaired securities				206	3,875
Gain on sales and calls of securities			(1,006)	(5,104)	(526)
Other non-cash interest income			(1,740)	(1,272)	(332)
Amortization of security premiums, net			2,945	6,371	5,140
Excess tax short-fall from stock options					5,348
Stock based and stock issued to officers and directors compensation expense			5,734	4,963	4,950
Net change in accrued interest receivable and other assets			25,747	13,478	(3,429)
Gain from acquisition			(5,628)
Net change in other liabilities			(2,641)	(2,784)	(15,506)
Net cash provided by operating activities			248,874	234,402	176,697
Cash Flows from Investing Activities
Decrease/(increase) in short-term investments and interest bearing deposits			796,322	(430,187)	(47,266)
Purchase of investment securities available-for-sale			(339,814)	(941,327)	(295,497)
Proceeds from maturity and call of investment securities available-for-sale			490,950	460,000	165,000
Proceeds from sale of investment securities available-for-sale			111,704	294	385,234
Purchase of mortgage-backed securities available-for-sale			(267,760)		(1,280,870)
Proceeds from repayment and sale of mortgage-backed securities available-for-sale			71,645	758,271	749,219
Purchase of Federal Home Loan Bank stock			(8,160)	(1,650)
Redemption of Federal Home Loan Bank stock			13,482	1,650	13,535
Redemption of Federal Reserve Bank stock			8,733
Net increase in loans			(963,858)	(1,051,952)	(829,501)
Purchase of premises and equipment			(3,188)	(3,523)	(3,518)
Proceeds from sales of premises and equipment			5,598	12	602
Proceeds from sales of other real estate owned			18,357	7,699	12,154
Increase in investment in affordable housing and alternative energy partnerships			(40,284)	(82,966)	(53,235)
Acquisitions, net of cash acquired			(118,392)		6,572
Net cash used in investing activities			(224,665)	(1,283,679)	(1,177,571)
Cash Flows from Financing Activities
Net increase in deposits			201,224	1,166,044	1,305,255
Net decrease in federal funds purchased and securities sold under agreements to repurchase			(250,000)	(50,000)	(50,000)
Advances from Federal Home Loan Bank			4,823,000	3,555,000	5,092,000
Repayment of Federal Home Loan Bank borrowings			(4,773,000)	(3,480,000)	(5,242,000)
Cash dividends			(69,888)	(59,274)	(45,283)
Purchase of treasury stock				(54,441)	(59,412)
Proceeds from issuance of long-term debt and other borrowings			75,000
Proceeds from shares issued to Dividend Reinvestment Plan			2,528	2,277	4,175
Proceeds from exercise of stock options			1,094	7,661	5,014
Taxes paid related to net share settlement of RSUs			(5,128)	(103)	(227)
Excess tax short-fall from share-based payment arrangements					(5,348)
Net cash provided by financing activities			4,830	1,087,164	1,004,174
Increase in cash and cash equivalents			29,039	37,887	3,300
Cash and cash equivalents, beginning of the year			218,017	180,130	176,830
Cash and cash equivalents, end of the year	247,056	218,017	247,056	218,017	180,130
Supplemental disclosure of cash flow information
Interest			81,069	81,793	72,870
Income taxes			76,709	38,671	69,074
Non-cash investing and financing activities:
Net change in unrealized holding gain/(loss) on securities available-for-sale, net of tax			485	3,886	(2,259)
Net change in unrealized gain/(loss) on interest rate swaps designated as cash flow hedges			719	825	(598)
Transfers to other real estate owned from loans held for investment			1,243	2,698	866
Loans transferred to loans held for sale			8,000	7,953	6,684
Loans to facilitate the sale of other real estate owned			10,500	2,616
Issuance of stock related to acquisition			34,862		82,857
Supplemental disclosure for acquisitions
Goodwill	372,189	372,189	372,189	372,189
Business Acquired During the Period [Member]
Supplemental disclosure for acquisitions
Cash and cash equivalents	166,932		166,932		63,579
Short-term investments	122,000		122,000
Securities available-for-sale	88,044		88,044		2,370
FHLB and FRB stock	19,890		19,890
Loans	705,792		705,792		419,219
Premises and equipment	6,239		6,239		13,291
Other real estate owned					3,048
Cash surrender value of life insurance	46,083		46,083
Deferred tax assets, net	40,690		40,690
Goodwill					55,849
Core deposit intangible	6,122		6,122		1,302
Accrued interest receivable and other assets	10,689		10,689		2,884
Total assets acquired	1,212,481		1,212,481		561,542
Deposits	813,888		813,888		420,623
Advances from Federal Home Loan Bank	30,000		30,000
Accrued interest payable and other liabilities	8,512		8,512		1,056
Total liabilities assumed	852,400		852,400		421,679
Net assets acquired	$ 360,081		360,081		139,863
Cash paid			285,324		57,006
Fair value of common stock issued			34,862		82,857
Total consideration paid			$ 320,186		$ 139,863