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	89,862
Other comprehensive income (loss), net of tax	17,973
Common stock dividends paid	(47,099)
Shares withheld to pay taxes, long-term incentive plan	(1,659)
Compensation expense, long-term incentive plan	3,250
Balance at Sep. 30, 2017	1,582,535
Balance at Dec. 31, 2017	1,571,701
Statement Of Stockholders Equity [Abstract]
Net income per consolidated statements of income	112,896
Other comprehensive income (loss), net of tax	(31,697)
Common stock dividends paid	(47,014)
Shares withheld to pay taxes, long-term incentive plan	(1,427)
Repurchase and retirement of common stock	(7,877)
Compensation expense, long-term incentive plan	3,022
Balance at Sep. 30, 2018	$ 1,599,604