Bank segment - Contractual maturities of securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less	$ 47,046
Due after one year through five years	89,085
Due after five years through ten years	37,911
Due after ten years	15,427
Total amortized cost	189,469
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	1,017,256
Amortized cost	1,206,725	$ 1,421,897
Held-to-Maturity, Debt Securities, Amortized Cost [Abstract]
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	132,704
Amortized cost	132,704	141,875
Available-for-sale Securities, Debt Securities, Fair Value [Abstract]
Due in one year or less	47,021
Due after one year through five years	90,675
Due after five years through ten years	38,320
Due after ten years	15,427
Total fair value	191,443
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	1,019,305
Total available-for-sale securities	1,210,748	1,388,533
Held-to-maturity Securities, Debt Securities, Fair Value [Abstract]
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	137,497
Total held-to-maturity securities	$ 137,497	$ 142,057