Bank segment - Contractual maturities of securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less	$ 25,004
Due after one year through five years	108,364
Due after five years through ten years	82,720
Due after ten years	27,518
Total amortized cost	243,606
Mortgage-related securities-FNMA, FHLMC and GNMA	1,195,492
Amortized cost	1,439,098	$ 1,421,622
Held-to-Maturity, Debt Securities, Amortized Cost [Abstract]
Mortgage-related securities-FNMA, FHLMC and GNMA	102,498
Amortized cost	102,498	44,515
Available-for-sale Securities, Debt Securities, Fair Value [Abstract]
Due in one year or less	24,896
Due after one year through five years	106,774
Due after five years through ten years	80,439
Due after ten years	26,772
Total fair value	238,881
Mortgage-related securities-FNMA, FHLMC and GNMA	1,148,690
Total available-for-sale securities	1,387,571	1,401,198
Held-to-maturity Securities, Debt Securities, Fair Value [Abstract]
Mortgage-related securities-FNMA, FHLMC and GNMA	99,929
Total held-to-maturity securities	$ 99,929	$ 44,412

Available-for-sale Securities, Debt Securities, Fair Value [Abstract]

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