Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]
Balance at beginning of period at Dec. 31, 2016	$ 2,571,617	$ 106,882	$ 2,125	$ 2,234,366	$ 304,397	$ (61,369)	$ (14,784)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	27,379				22,979	4,400
Dividends declared:
Preferred stock	(2,010)				(2,010)
Common stock: $0.12/share	(25,548)				(25,548)
Issuance of common stock	(803)		5	2,117			(2,925)
Issuance of common stock - acquisitions	1,782,007		1,116	1,780,891
Restricted stock compensation	1,394			1,394
Tax benefit of stock-based compensation	1,759			1,759
Balance at end of period at Mar. 31, 2017	4,355,795	106,882	3,246	4,020,527	299,818	(56,969)	(17,709)
Balance at beginning of period at Dec. 31, 2017	4,409,194	106,882	3,253	4,033,567	367,658	(83,052)	(19,114)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	61,090				86,762	(25,672)
Dividends declared:
Preferred stock	(2,010)				(2,010)
Common stock: $0.12/share	(39,070)				(39,070)
Issuance of common stock	2,310		2	2,341			(33)
Restricted stock compensation	1,939			1,939
Balance at end of period at Mar. 31, 2018	$ 4,433,453	$ 106,882	$ 3,255	$ 4,037,847	$ 413,340	$ (108,724)	$ (19,147)