Consolidated Statements of Changes in Shareholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2016	$ 1,814,417	$ 135,159	$ 1,348,338	$ 390,292	$ (59,372)
Net income	74,846			74,846
Other comprehensive income (loss)	28,252				28,252
Dividends - common stock	(35,219)			(35,219)
Common stock issued	188	11	177
Common stock repurchased	(1,852)	(104)	(1,748)
Share-based compensation expense	3,184		3,184
Stock activity under incentive compensation plans	2,778	450	2,460	(132)
Ending Balance at Jun. 30, 2017	1,886,594	135,516	1,352,411	429,787	(31,120)
Beginning Balance at Mar. 31, 2017					(48,565)
Net income	38,854
Other comprehensive income (loss)	17,445
Ending Balance at Jun. 30, 2017	1,886,594	135,516	1,352,411	429,787	(31,120)
Beginning Balance at Dec. 31, 2017	2,154,397	152,040	1,639,499	413,130	(50,272)
Reclassification of certain tax effects related to the Tax Cuts and Jobs Act of 2017 (Note 3)				10,751	(10,751)
Net income	91,984			91,984
Other comprehensive income (loss)	(5,628)				(5,628)
Dividends - common stock	(39,588)			(39,588)
Common stock issued	222	14	208
Common stock repurchased	(1,749)	(100)	(1,649)
Share-based compensation expense	3,606		3,606
Stock activity under incentive compensation plans	1,150	397	1,126	(373)
Ending Balance at Jun. 30, 2018	2,200,215	152,351	1,642,790	471,777	(66,703)
Beginning Balance at Mar. 31, 2018					(61,526)
Net income	44,001
Other comprehensive income (loss)	(5,177)
Ending Balance at Jun. 30, 2018	2,200,215	$ 152,351	$ 1,642,790	471,777	(66,703)
Cumulative effect of change in accounting principles (Note 3)	$ (4,179)			$ (4,127)	$ (52)