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	88,180
Other comprehensive income, net of tax	13,904
Common stock dividends paid	(46,952)
Common stock issued-net, long-term incentive plan	(842)
Excess tax expense from stock-based compensation arrangements	(212)
Compensation expense, long-term incentive plan	2,738
Balance at Sep. 30, 2015	1,476,756
Balance at Dec. 31, 2015	1,473,057
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	79,488
Other comprehensive income, net of tax	28,319
Common stock dividends paid	(46,983)
Common stock issued-net, long-term incentive plan	(992)
Repurchase and retirement of common stock	(750)
Excess tax expense from stock-based compensation arrangements	(119)
Compensation expense, long-term incentive plan	2,741
Balance at Sep. 30, 2016	$ 1,534,761