Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Treasury Stock [Member]
Total shareholders’ equity at beginning of period at Dec. 31, 2017	$ 3,297,863	$ 144,486	$ 642	$ 953,361	$ 2,187,069	$ 79,512	$ (67,207)
Cumulative effect of accounting change at Dec. 31, 2017	(2,285)				(2,285)
Total shareholders' equity at beginning of period, as adjusted at Dec. 31, 2017	3,295,578	144,486	642	953,361	2,184,784	79,512	(67,207)
Net income	106,480				106,480
Other comprehensive income (loss)	(142,401)					(142,401)
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act					(9,535)	9,535
Stock option exercises/stock unit conversions (100,865 shares YTD in 2019 and 318,110 shares YTD in 2018)	19,165				(8,861)		28,026
Stock compensation expense recognized in earnings	3,175			3,175
Cash dividends – preferred stock (approximately $0.34 per share YTD in both 2019 and in 2018)	(2,016)				(2,016)
Cash dividends – common stock ($0.67 per share YTD in 2019 and $0.57 per share YTD in 2018)	(36,551)				(36,551)
Total shareholders’ equity at end of period at Mar. 31, 2018	3,243,430	144,486	642	956,536	2,234,301	(53,354)	(39,181)
Total shareholders’ equity at beginning of period at Dec. 31, 2018	3,368,917	144,486	642	967,304	2,440,002	(63,600)	(119,917)
Cumulative effect of accounting change at Dec. 31, 2018	(12,611)				(12,611)
Total shareholders' equity at beginning of period, as adjusted at Dec. 31, 2018	3,356,306	144,486	642	967,304	2,427,391	(63,600)	(119,917)
Net income	116,496				116,496
Other comprehensive income (loss)	157,374					157,374
Stock option exercises/stock unit conversions (100,865 shares YTD in 2019 and 318,110 shares YTD in 2018)	4,885				(4,001)		8,886
Stock compensation expense recognized in earnings	3,654			3,654
Purchase of treasury stock (5,351 shares YTD in 2019 and no shares YTD in 2018)	(519)						(519)
Cash dividends – preferred stock (approximately $0.34 per share YTD in both 2019 and in 2018)	(2,016)				(2,016)
Cash dividends – common stock ($0.67 per share YTD in 2019 and $0.57 per share YTD in 2018)	(42,602)				(42,602)
Total shareholders’ equity at end of period at Mar. 31, 2019	$ 3,593,578	$ 144,486	$ 642	$ 970,958	$ 2,495,268	$ 93,774	$ (111,550)