Consolidated Condensed Statements of Changes in Shareholders' Equity (Unaudited) $ in Thousands	USD ($)
Balance at Dec. 31, 2014	$ 1,419,940
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	29,148
Other comprehensive income, net of tax	12,832
Common stock dividends paid	(15,639)
Common stock issued-net, long-term incentive plan	(830)
Excess tax expense from stock-based compensation arrangements	(218)
Compensation expense, long-term incentive plan	851
Balance at Mar. 31, 2015	1,446,084
Balance at Dec. 31, 2015	1,473,057
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	27,003
Other comprehensive income, net of tax	23,821
Common stock dividends paid	(15,640)
Common stock issued-net, long-term incentive plan	(799)
Excess tax expense from stock-based compensation arrangements	(147)
Compensation expense, long-term incentive plan	961
Balance at Mar. 31, 2016	$ 1,508,256