Parent Company Financial Information - STATEMENTS OF FINANCIAL CONDITION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Information Disclosure [Abstract]
Percentage of capital stock of the Bank held	100.00%
Assets
Cash and cash equivalents	$ 203,406	$ 197,164	$ 156,857	$ 78,417
Other assets	75,667	52,264
Total assets	11,487,387	8,024,501
Liabilities
Subordinated debentures	110,313	105,123
Accrued expenses and other liabilities	81,345	55,209
Total liabilities	9,517,690	6,782,505
Total Stockholders’ Equity	1,969,697	1,241,996	459,740	298,980
Total liabilities and stockholders’ equity	11,487,387	8,024,501
Corporation
Assets
Cash and cash equivalents	13,160	17,097	$ 15,124	$ 3,412
Investment in subsidiaries	2,068,077	1,329,961
Other assets	1,689	2,599
Total assets	2,082,926	1,349,657
Liabilities
Subordinated debentures	110,313	105,123
Accrued expenses and other liabilities	2,916	2,538
Total liabilities	113,229	107,661
Total Stockholders’ Equity	1,969,697	1,241,996
Total liabilities and stockholders’ equity	$ 2,082,926	$ 1,349,657