Parent Company Financial Information - STATEMENTS OF FINANCIAL CONDITION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Percentage of capital stock of the Bank held	100.00%
Assets
Cash and cash equivalents	$ 200,064	$ 156,857	$ 78,417	$ 110,925
Other assets	52,067	25,874
Total assets	8,024,501	4,036,311
Liabilities
Subordinated debentures	105,123	69,383
Accrued expenses and other liabilities	55,227	33,636
Total liabilities	6,782,505	3,576,571
Total Stockholders’ Equity	1,241,996	459,740	298,980	199,592
Total liabilities and stockholders' equity	8,024,501	4,036,311
Corporation
Assets
Cash and cash equivalents	17,097	15,124	$ 3,412	$ 18,724
Investment in subsidiaries	1,329,961	513,606
Other assets	2,599	2,400
Total assets	1,349,657	531,130
Liabilities
Subordinated debentures	105,123	69,383
Accrued expenses and other liabilities	2,538	2,007
Total liabilities	107,661	71,390
Total Stockholders’ Equity	1,241,996	459,740
Total liabilities and stockholders' equity	$ 1,349,657	$ 531,130