Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 162,954	$ 161,496	$ 165,128	$ 155,515	$ 146,453	$ 142,615	$ 135,670	$ 119,546	$ 645,093	$ 544,284	$ 400,532
Interest expense	113,584	121,384	122,074	114,916	104,237	97,557	91,737	76,317	471,958	369,848	242,885
Net interest income	49,370	40,112	43,054	40,599	42,216	45,058	43,933	43,229	173,135	174,436	157,647
(Provision for)/release of loan losses	(2,430)	(760)	(578)	264	(146)	(99)	(424)	431	(3,504)	(238)	(1,708)
Net interest income after (provision for)/release of loan losses	46,940	39,352	42,476	40,863	42,070	44,959	43,509	43,660	169,631	174,198	155,939
Guarantee and commitment fees	3,401	3,349	3,403	3,513	3,506	3,490	3,481	3,499	13,666	13,976	14,114
Gains/(losses) on financial derivatives	4,089	(7,360)	8,913	(360)	(2,999)	628	2,534	(3,850)
Gains on trading assets	172	49	61	44	57	(3)	11	16	326	81	(24)
(Losses)/gains on sale of available-for-sale investment securities	(236)	0	0	0					(236)	0	89
(Losses)/gains on sale of real estate owned					0	(41)	34	0	0	(7)	1,748
Other income	526	530	355	493	118	365	320	574	1,904	1,377	832
Non-interest income	7,952	(3,432)	12,732	3,690	682	4,439	6,380	239	20,942	11,740	17,512
Non-interest expense	13,651	13,458	12,052	12,761	13,703	11,650	12,921	11,642	51,922	49,916	42,765
Income before income taxes	41,241	22,462	43,156	31,792	29,049	37,748	36,968	32,257	138,651	136,022	130,686
Income tax expense	8,743	4,629	9,111	6,622	6,193	7,979	7,332	6,438	29,105	27,942	46,369
Net income	32,498	17,833	34,045	25,170	22,856	29,769	29,636	25,819	109,546	108,080	84,482
Preferred stock dividends	(3,432)	(3,427)	(3,785)	(3,296)	(3,296)	(3,295)	(3,296)	(3,295)	(13,940)	(13,182)	(13,182)
Loss on retirement of preferred stock	0	0	(1,956)	0					(1,956)	0	0
Net income attributable to common stockholders	$ 29,066	$ 14,406	$ 28,304	$ 21,874	$ 19,560	$ 26,474	$ 26,340	$ 22,524	$ 93,650	$ 94,898	$ 71,300
Basic earnings per common share (in dollars per share)	$ 2.72	$ 1.34	$ 2.65	$ 2.05	$ 1.84	$ 2.48	$ 2.47	$ 2.12	$ 8.76	$ 8.91	$ 6.73
Diluted earnings per common share (in dollars per share)	$ 2.70	$ 1.33	$ 2.63	$ 2.03	$ 1.82	$ 2.46	$ 2.45	$ 2.10	$ 8.69	$ 8.83	$ 6.60