Investment Securities - The amortized cost and estimated fair value of securities (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) purchase_agreement	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)
Investment securities available-for-sale
Amortized Cost		$ 786,783,000	$ 385,662,000
Unrealized Gain		6,161,000	804,000
Unrealized Loss		(5,515,000)	(5,503,000)
Estimated Fair Value		787,429,000	380,963,000
Investment securities held-to-maturity
Amortized Cost		18,291,000	8,565,000
Unrealized Gain		0	0
Unrealized Loss		(209,000)	(104,000)
Estimated Fair Value		18,082,000	8,461,000
Total investment securities
Amortized Cost		805,074,000	394,227,000
Unrealized Gain		6,161,000	804,000
Unrealized Loss		(5,724,000)	(5,607,000)
Estimated Fair Value		805,511,000	389,424,000
Additional disclosures
Accumulated other comprehensive income (loss)		646,000	(4,700,000)
Accumulated other comprehensive income (loss), net of tax		$ 415,000	(2,721,000)
Number of inverse putable reverse repurchase of the Bank's secured by collateral | purchase_agreement		3
Value of inverse putable reverse repurchases secured by collateral		$ 28,500,000
OTTI realized		2,000	205,000
Gross gains		3,100,000	1,800,000	$ 317,000
Gross losses		(386,000)	(9,000)	(27,000)
Net proceeds from the sale of available-for-sale securities		268,596,000	230,945,000	$ 27,642,000
CRA
Investment securities available-for-sale
Estimated Fair Value					$ 500,000
Additional disclosures
OTTI realized	$ 207,000
CRA investment purchased, par value					$ 50
Mortgage-backed securities: residential
Additional disclosures
Estimated par value of securities pledged as collateral for the Bank's inverse putable reverse repurchases		55,600,000
Fair value of securities pledged as collateral for the Bank's inverse putable reverse repurchases		57,000,000
Home Owners Association Loans
Additional disclosures
Value of inverse putable reverse repurchases secured by collateral		17,600,000
Agency
Investment securities available-for-sale
Amortized Cost		47,051,000
Unrealized Gain		236,000
Unrealized Loss		(78,000)
Estimated Fair Value		47,209,000
Corporate
Investment securities available-for-sale
Amortized Cost		78,155,000	37,475,000
Unrealized Gain		1,585,000	372,000
Unrealized Loss		(194,000)	(205,000)
Estimated Fair Value		79,546,000	37,642,000
Municipal bonds
Investment securities available-for-sale
Amortized Cost		228,929,000	120,155,000
Unrealized Gain		3,942,000	338,000
Unrealized Loss		(743,000)	(1,690,000)
Estimated Fair Value		232,128,000	118,803,000
Collateralized mortgage obligation: residential
Investment securities available-for-sale
Amortized Cost		33,984,000	31,536,000
Unrealized Gain		132,000	25,000
Unrealized Loss		(335,000)	(173,000)
Estimated Fair Value		33,781,000	31,388,000
Mortgage-backed securities: residential
Investment securities available-for-sale
Amortized Cost		398,664,000	196,496,000
Unrealized Gain		266,000	69,000
Unrealized Loss		(4,165,000)	(3,435,000)
Estimated Fair Value		394,765,000	193,130,000
Investment securities held-to-maturity
Amortized Cost		17,153,000	7,375,000
Unrealized Gain		0	0
Unrealized Loss		(209,000)	(104,000)
Estimated Fair Value		16,944,000	7,271,000
Other
Investment securities held-to-maturity
Amortized Cost		1,138,000	1,190,000
Unrealized Gain		0	0
Unrealized Loss		0	0
Estimated Fair Value		$ 1,138,000	$ 1,190,000