Bank segment - Contractual maturities of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less	$ 15,000
Due after one year through five years	83,983
Due after five years through ten years	69,986
Due after ten years	28,377
Total amortized cost	197,346
Mortgage-related securities-FNMA, FHLMC and GNMA	1,259,732
Amortized cost	1,457,078	$ 1,421,622
Held-to-Maturity, Debt Securities, Amortized Cost [Abstract]
Mortgage-related securities-FNMA, FHLMC and GNMA	43,450
Amortized cost	43,450	44,515
Available-for-sale Securities, Debt Securities, Fair Value [Abstract]
Due in one year or less	14,902
Due after one year through five years	82,887
Due after five years through ten years	68,521
Due after ten years	27,945
Total fair value	194,255
Mortgage-related securities-FNMA, FHLMC and GNMA	1,224,235
Total available-for-sale securities	1,418,490	1,401,198
Held-to-maturity Securities, Debt Securities, Fair Value [Abstract]
Mortgage-related securities-FNMA, FHLMC and GNMA	42,491
Total held-to-maturity securities	$ 42,491	$ 44,412