Bank segment - Contractual maturities of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less	$ 60,299
Due after one year through five years	70,894
Due after five years through ten years	49,085
Due after ten years	15,427
Total amortized cost	195,705
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	1,114,580
Amortized cost	1,310,285	$ 1,229,438
Held-to-Maturity, Debt Securities, Amortized Cost [Abstract]
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	134,656
Amortized cost	134,656	139,451
Available-for-sale Securities, Debt Securities, Fair Value [Abstract]
Due in one year or less	60,622
Due after one year through five years	73,071
Due after five years through ten years	49,669
Due after ten years	15,427
Total fair value	198,789
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	1,141,452
Total available-for-sale securities	1,340,241	1,232,826
Held-to-maturity Securities, Debt Securities, Fair Value [Abstract]
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	142,570
Total held-to-maturity securities	$ 142,570	$ 143,467