Junior Subordinated Debentures - Schedule of Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 257.0	$ 256.6
Argo Group [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	28.4	28.4
Trust Preferred Debentures [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 172.7	$ 172.7
Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Statutory Trust II [Member]
Debt Instrument [Line Items]
Interest Rate	6.72%	5.52%
Junior subordinated debentures, Amount	$ 18.1	$ 18.1
Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Trust VI [Member]
Debt Instrument [Line Items]
Interest Rate	6.70%	5.59%
Junior subordinated debentures, Amount	$ 10.3	$ 10.3
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust I [Member]
Debt Instrument [Line Items]
Interest Rate	6.72%	5.52%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust III [Member]
Debt Instrument [Line Items]
Interest Rate	6.54%	5.46%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IV [Member]
Debt Instrument [Line Items]
Interest Rate	6.47%	5.27%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust V [Member]
Debt Instrument [Line Items]
Interest Rate	6.54%	5.30%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VI [Member]
Debt Instrument [Line Items]
Interest Rate	6.59%	5.40%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VII [Member]
Debt Instrument [Line Items]
Interest Rate	6.39%	5.19%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VIII [Member]
Debt Instrument [Line Items]
Interest Rate	6.37%	5.21%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IX [Member]
Debt Instrument [Line Items]
Interest Rate	6.39%	5.19%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust X [Member]
Debt Instrument [Line Items]
Interest Rate	6.19%	4.99%
Junior subordinated debentures, Amount	$ 30.9	$ 30.9
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Statutory Trust II [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Trust VI [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.90%	3.90%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust I [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust III [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IV [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust V [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VI [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.80%	3.80%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VII [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VIII [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.55%	3.55%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IX [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust X [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.40%	3.40%