Investments in unconsolidated entities - Equity method unconsolidated entities (Details) - Investments in unconsolidated entities - Equity method eligible investments	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
BE Reinsurance Limited
Investments in unconsolidated entities
Ownership interest	25.00%	25.00%
BioVentures Investors (Offshore) IV LP
Investments in unconsolidated entities
Ownership interest	73.00%	73.00%
Camden Partners Strategic Fund V (Cayman), LP
Investments in unconsolidated entities
Ownership interest	39.40%	36.40%
NEC Cypress Buyer LLC
Investments in unconsolidated entities
Ownership interest	23.50%	13.30%
New Energy Capital Infrastructure Credit Fund LP
Investments in unconsolidated entities
Ownership interest	18.30%	22.90%
New Energy Capital Infrastructure Offshore Credit Fund LP
Investments in unconsolidated entities
Ownership interest	12.20%	54.90%
Scion G7, LP
Investments in unconsolidated entities
Ownership interest	28.30%	28.80%
Tuckerman Capital V LP
Investments in unconsolidated entities
Ownership interest	48.30%	47.60%
Tuckerman Capital V Co-Investment I LP
Investments in unconsolidated entities
Ownership interest	49.50%	47.70%