Fair value measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	$ 1,105,182	$ 820,648
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Weighted average discount rate	2.517%
Mortgage-related securities-FNMA, FHLMC and GNMA
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	$ 897,474	607,689
U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	192,281	212,959
Mortgage revenue bond
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	15,427
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2015	0
Principal payments received	0
Purchases	15,427
Unrealized gain (loss) included in other comprehensive income	0
Balance at December 31, 2016	15,427	0
Fair value measurements on a recurring basis \| Level 1
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	129	15
Fair value measurements on a recurring basis \| Level 1 \| Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 1 \| Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	129	15
Fair value measurements on a recurring basis \| Level 1 \| Foreign Exchange Forward
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 1 \| Other
Fair value measurements on a recurring and nonrecurring basis
Money market funds	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Mortgage-related securities-FNMA, FHLMC and GNMA
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Mortgage revenue bond
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 2
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	453	385
Derivative liabilities	823	15
Fair value measurements on a recurring basis \| Level 2 \| Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	445	384
Derivative liabilities	24	0
Fair value measurements on a recurring basis \| Level 2 \| Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	8	1
Derivative liabilities	56	15
Fair value measurements on a recurring basis \| Level 2 \| Foreign Exchange Forward
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	743	0
Fair value measurements on a recurring basis \| Level 2 \| Other
Fair value measurements on a recurring and nonrecurring basis
Money market funds	13,085	10
Fair value measurements on a recurring basis \| Level 2 \| Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	1,089,755	820,648
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Mortgage-related securities-FNMA, FHLMC and GNMA
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	897,474	607,689
Fair value measurements on a recurring basis \| Level 2 \| Bank \| U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	192,281	212,959
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Mortgage revenue bond
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 3
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Foreign Exchange Forward
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Other
Fair value measurements on a recurring and nonrecurring basis
Money market funds	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	15,427	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Mortgage-related securities-FNMA, FHLMC and GNMA
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Mortgage revenue bond
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale investment securities, at fair value	$ 15,427	$ 0

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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