Summary of Net Assets Acquired (at Fair Value) and Consideration Transferred (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	May. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Total assets acquired		$ 1,321,453
Total liabilities assumed		1,159,920
Preliminary goodwill		$ 227,962	$ 209,758	$ 209,758
Marquette [Member]
Business Acquisition [Line Items]
Loans	$ 980,404
Investment securities	177,694
Cash and due from banks	104,610
Premises and equipment, net	11,508
Identifiable intangible assets	14,881
Other assets	32,356
Total assets acquired	1,321,453
Noninterest-bearing deposits	235,426
Interest-bearing deposits	708,675
Short-term debt	112,133
Long-term debt	89,971
Other liabilities	13,715
Total liabilities assumed	1,159,920
Net identifiable assets acquired	161,533
Preliminary goodwill	18,204
Net assets acquired	$ 179,737
Company's common shares issued	3,470
Purchase price per share of the Company's common stock	$ 51.79
Fair value of total consideration transferred	$ 179,737