Consolidated Statements of Changes in Shareholders' Equity (Unaudited) $ in Thousands	USD ($)
Balance at Dec. 31, 2013	$ 1,354,953
Consolidated Statements of Changes in Shareholders' Equity (Unaudited) [Abstract]
Net income per consolidated statements of income	95,489
Other comprehensive income, net of tax	9,366
Common stock dividends paid	(46,852)
Common stock issued-net, long-term incentive plans	(795)
Excess tax expense from stock-based compensation arrangements	(353)
Compensation expense, long-term incentive plans	3,290
Balance at Sep. 30, 2014	1,415,098
Balance at Dec. 31, 2014	1,419,940
Consolidated Statements of Changes in Shareholders' Equity (Unaudited) [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 plans	(842)
Excess tax expense from stock-based compensation arrangements	(212)
Compensation expense, long-term incentive plans	2,738
Balance at Sep. 30, 2015	$ 1,476,756