Consolidated Condensed Statements of Changes in Shareholders' Equity (Unaudited) $ in Thousands	USD ($)
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
Balance at Dec. 31, 2016	1,520,208
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	31,248
Other comprehensive income, net of tax	2,793
Common stock dividends paid	(15,697)
Common stock issued-net, long-term incentive plan	(1,543)
Compensation expense, long-term incentive plan	952
Balance at Mar. 31, 2017	$ 1,537,961