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	114,218			114,218
Other comprehensive income (loss)	25,175				25,175
Dividends - common stock	(52,841)			(52,841)
Common stock issued	303	18	285
Common stock repurchased	(1,917)	(108)	(1,809)
Share-based compensation expense	4,684		4,684
Stock activity under incentive compensation plans	2,784	454	2,538	(208)
Ending Balance at Sep. 30, 2017	1,906,823	135,523	1,354,036	451,461	(34,197)
Beginning Balance at Jun. 30, 2017					(31,120)
Net income	39,372
Other comprehensive income (loss)	(3,077)
Ending Balance at Sep. 30, 2017	1,906,823	135,523	1,354,036	451,461	(34,197)
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	143,332			143,332
Other comprehensive income (loss)	(18,855)				(18,855)
Dividends - common stock	(59,394)			(59,394)
Common stock issued	371	22	349
Common stock repurchased	(1,776)	(102)	(1,674)
Share-based compensation expense	5,629		5,629
Stock activity under incentive compensation plans	1,155	392	1,308	(545)
Ending Balance at Sep. 30, 2018	2,220,680	152,352	1,645,111	503,147	(79,930)
Beginning Balance at Jun. 30, 2018					(66,703)
Net income	51,348
Other comprehensive income (loss)	(13,227)
Ending Balance at Sep. 30, 2018	2,220,680	$ 152,352	$ 1,645,111	503,147	(79,930)
Cumulative effect of change in accounting principles (Note 3)	$ (4,179)			$ (4,127)	$ (52)