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	79,488
Other comprehensive income (loss), 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
Balance at Dec. 31, 2016	1,520,208
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	89,862
Other comprehensive income (loss), net of tax	17,973
Common stock dividends paid	(47,099)
Common stock issued-net, long-term incentive plan	(1,659)
Compensation expense, long-term incentive plan	3,250
Balance at Sep. 30, 2017	$ 1,582,535