Businss Combinations (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Dec. 18, 2017	Sep. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	May 05, 2017	Jan. 13, 2017	Sep. 01, 2016	May 06, 2016	Mar. 11, 2016
Business Acquisition [Line Items]
Total assets			$ 34,527,512	$ 32,990,836
Deposits			29,266,275	28,161,343
Loans and Leases			23,596,825	21,737,878
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			199,682	80,690
Total purchase price	$ 113,600
Gain on acquisitions			134,745	5,831	$ 42,930
Merger-related expenses			9,015	5,341	14,174
Goodwill			150,601	150,601	$ 139,773
HomeBancorp Inc. [Member]
Business Acquisition [Line Items]
Business Acquisition, Share Price	$ 15.03
Total assets						$ 954,900
Deposits						699,400
Loans and Leases						$ 637,500
Cordia Bancorp Inc.
Business Acquisition [Line Items]
Business Acquisition, Share Price									$ 5.15
Total purchase price		$ 37,053
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value									$ 241,392
Other intangible assets									2,210
Total assets acquired									349,328
Deposits									292,192
Total liabilities assumed									323,103
Goodwill acquired			0	10,828
Revenue of acquired business			5,561	4,226
Merger-related expenses			260	3,810
Goodwill									$ 10,828
First CornerStone Bank
Business Acquisition [Line Items]
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value										$ 43,776
Other intangible assets										390
Total assets acquired										87,414
Gain on acquisitions				2,959
Total liabilities assumed										96,905
Fair value of net assets (liabilities) acquired										(9,491)
Business Combination Cash Received From FDIC In Acquisition										$ 12,450
Revenue of acquired business			1,700	1,600
Merger-related expenses				1,000
North Milwaukee State Bank
Business Acquisition [Line Items]
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value											$ 36,914
Other intangible assets											240
Total assets acquired											53,614
Deposits											59,206
Gain on acquisitions				2,872
Total liabilities assumed											60,942
Fair value of net assets (liabilities) acquired											7,328
Business Combination Cash Received From FDIC In Acquisition											$ 10,200
Revenue of acquired business			2,400	1,900
Merger-related expenses			100	$ 500
Guaranty Bank
Business Acquisition [Line Items]
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value							$ 114,533
Cash and due from banks							48,824
Overnight investments							94,134
Investment securities							12,140
Loans and leases							689,086
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			574,553				574,553
Premises and equipment							8,603
Income earned not collected							6,720
Other intangible assets							9,870
Other assets							5,748
Total assets acquired							875,125
Deposits							982,307
Gain on acquisitions			122,728
Other liabilities							440
Total liabilities assumed							982,747
Fair value of net assets (liabilities) acquired							(107,622)
Business Combination Cash Received From FDIC In Acquisition							$ 230,350
Revenue of acquired business			20,501
Merger-related expenses			7,391
Harvest Community Bank
Business Acquisition [Line Items]
Certain Loans Acquired in Transfer Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value								$ 85,149
Cash and due from banks								3,350
Overnight investments								7,478
Investment securities								14,455
Income earned not collected								31
Other intangible assets								850
Other assets								237
Total assets acquired								111,550
Deposits								121,755
Gain on acquisitions			12,017
Other liabilities								74
Total liabilities assumed								121,829
Fair value of net assets (liabilities) acquired								(10,279)
Business Combination Cash Received From FDIC In Acquisition								$ 22,296
Revenue of acquired business			3,799
Merger-related expenses			$ 1,192