Bank segment - Contractual maturities of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less	$ 15,000
Due after one year through five years	133,142
Due after five years through ten years	55,997
Due after ten years	15,427
Total amortized cost	219,566
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	1,149,167
Amortized cost	1,368,733	$ 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	140,203
Amortized cost	140,203	141,875
Available-for-sale Securities, Debt Securities, Fair Value [Abstract]
Due in one year or less	14,960
Due after one year through five years	133,294
Due after five years through ten years	55,595
Due after ten years	15,427
Total fair value	219,276
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	1,128,987
Total available-for-sale securities	1,348,263	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	142,333
Total held-to-maturity securities	$ 142,333	$ 142,057