Stockholders' Equity (Details)								12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Nov. 07, 2014 USD ($) $ / shares shares	Nov. 22, 2013 USD ($) shares	Nov. 12, 2013 USD ($) shares	Jul. 02, 2013 USD ($) shares	Jan. 03, 2013 USD ($) shares	Sep. 22, 2011 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Class of Stock [Line Items]
Stockholders' equity	$ 850,509,000							$ 850,509,000	$ 726,827,000	$ 634,923,000	$ 552,236,000
Dividend reinvestment and stock purchase plan, stockholder's maximum amount of cash payments per quarter in purchase of common stock								$ 2,500
Preferred stock, liquidation value (in dollars per share) | $ / shares	$ 1,000							$ 1,000	$ 1,000
Price of preferred stock redeemed								$ 1,640,000	$ 1,067,000	491,000
Preferred stock, shares outstanding | shares	125							125	125
Value of stock issued as a part of acquisition								$ 70,402,000	$ 34,981,000	$ 135,642,000
Cash paid in acquisition								$ 14,500,000	$ 14,208,000
Merger with CFS Bancorp, Inc.
Class of Stock [Line Items]
Percentage of interest acquired				100.00%
Value of stock issued as a part of acquisition				$ 135,600,000
Percentage of common stock received per shares owned through acquisition				0.65%
Stock issued as a part of acquisition (in shares) | shares				7,100,000
Total purchase price				$ 135,642,000
Merger with Community Bancshares, Inc.
Class of Stock [Line Items]
Percentage of interest acquired		100.00%
Value of stock issued as a part of acquisition		$ 35,000,000
Stock issued as a part of acquisition (in shares) | shares		1,600,000
Number of shares of common stock		4.0926
Share price (in dollars per share) | $ / shares		$ 85.94
Cash paid in acquisition		$ 14,200,000
Total purchase price		$ 49,189,000
Merger with Ameriana Bancorp
Class of Stock [Line Items]
Percentage of interest acquired	100.00%							100.00%
Value of stock issued as a part of acquisition	$ 70,400,000
Stock issued as a part of acquisition (in shares) | shares	2,800,000
Total purchase price	$ 70,402,000							$ 70,402,000
Series B Preferred Stock
Class of Stock [Line Items]
Number of new stock issued during the period (shares) | shares							90,782.94
Preferred stock, liquidation value (in dollars per share) | $ / shares							$ 1,000
Number of new stock issued during the period value							$ 90,782,940
Preferred stock redeemed (in shares) | shares			34,043		34,044	22,695.94
Price of preferred stock redeemed			$ 34,043,000		$ 34,044,000	$ 22,695,940
Preferred stock, shares outstanding | shares	0				34,043	68,087		0
Series B Preferred Stock remaining liquidation amount					$ 34,000,000	$ 68,000,000
Common Stock | Merger with Ameriana Bancorp
Class of Stock [Line Items]
Number of shares of common stock	0.9037							0.9037
Subsidiaries
Class of Stock [Line Items]
Amount available for dividends	$ 29,812,000							$ 29,812,000
Stockholders' equity	930,360,000							930,360,000
Stockholders' equity restricted from dividend distribution	$ 900,548,000							$ 900,548,000
CFS Bancorp | Merger with CFS Bancorp, Inc.
Class of Stock [Line Items]
Percentage of common stock received per shares owned through acquisition				0.65%