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	61,900
Other comprehensive income, net of tax	13,174
Common stock dividends paid	(31,224)
Common stock issued-net, long-term incentive plan	(795)
Excess tax expense from stock-based compensation arrangements	(353)
Compensation expense, long-term incentive plans	2,236
Balance at Jun. 30, 2014	1,399,891
Balance at Dec. 31, 2014	1,419,940
Consolidated Statements of Changes in Shareholders' Equity (Unaudited) [Abstract]
Net income per consolidated statements of income	59,750
Other comprehensive income, net of tax	1,291
Common stock dividends paid	(31,294)
Common stock issued-net, long-term incentive plan	(842)
Excess tax expense from stock-based compensation arrangements	(217)
Compensation expense, long-term incentive plans	1,781
Balance at Jun. 30, 2015	$ 1,450,409