Consolidated Condensed Statements of Changes in Shareholders' Equity (Unaudited) $ in Thousands	USD ($)
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
Balance at Dec. 31, 2017	1,571,701
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	76,643
Other comprehensive income (loss), net of tax	(25,490)
Common stock dividends paid	(31,347)
Common stock issued-net, long-term incentive plan	(1,422)
Repurchase and retirement of common stock	(7,877)
Compensation expense, long-term incentive plan	1,864
Balance at Jun. 30, 2018	$ 1,584,072