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	48,506
Other comprehensive income (loss), net of tax	33,230
Common stock dividends paid	(31,301)
Common stock issued-net, long-term incentive plan	(949)
Repurchase and retirement of common stock	(750)
Excess tax expense from stock-based compensation arrangements	(126)
Compensation expense, long-term incentive plan	1,800
Balance at Jun. 30, 2016	1,523,467
Balance at Dec. 31, 2016	1,520,208
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	55,283
Other comprehensive income (loss), net of tax	17,289
Common stock dividends paid	(31,397)
Common stock issued-net, long-term incentive plan	(1,639)
Compensation expense, long-term incentive plan	2,174
Balance at Jun. 30, 2017	$ 1,561,918