Derivative Instruments - Interest Rate Swaps (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Fixed Income Interest Rate
Currency swaps and forwards
Notional Amount	$ 2,551,000	$ 3,127,800
Average Fixed Pay Rate	2.20%	2.30%
Average Floating Receive Rate	2.00%	2.60%
Average Maturity (Years)	7 years 1 month 6 days	6 years
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Variable Pay Rate
Currency swaps and forwards
Notional Amount	$ 1,405,000	$ 728,400
Average Fixed Pay Rate	2.00%	2.50%
Average Floating Receive Rate	1.90%	2.40%
Average Maturity (Years)	3 years 6 months	8 years 3 months 18 days
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | 1 year or less | Fixed Income Interest Rate
Currency swaps and forwards
Notional Amount	$ 200,000	$ 400,000
Average Fixed Pay Rate	1.80%	1.50%
Average Floating Receive Rate	1.90%	2.80%
Average Maturity (Years)	4 months 24 days	6 months
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Greater than 1 years and less than 3 years | Fixed Income Interest Rate
Currency swaps and forwards
Notional Amount		$ 200,000
Average Fixed Pay Rate		1.80%
Average Floating Receive Rate		2.60%
Average Maturity (Years)		1 year 4 months 24 days
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Greater than 1 years and less than 3 years | Variable Pay Rate
Currency swaps and forwards
Notional Amount	$ 810,000
Average Fixed Pay Rate	2.00%
Average Floating Receive Rate	2.00%
Average Maturity (Years)	1 year 7 months 6 days
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Greater than 3 years and less than 5 years | Fixed Income Interest Rate
Currency swaps and forwards
Notional Amount	$ 622,400	$ 1,104,700
Average Fixed Pay Rate	2.60%	2.30%
Average Floating Receive Rate	1.90%	2.50%
Average Maturity (Years)	4 years 1 month 6 days	3 years 9 months 18 days
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Greater than 3 years and less than 5 years | Variable Pay Rate
Currency swaps and forwards
Notional Amount	$ 550,000
Average Fixed Pay Rate	1.90%
Average Floating Receive Rate	1.60%
Average Maturity (Years)	5 years
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Greater than 5 years | Fixed Income Interest Rate
Currency swaps and forwards
Notional Amount	$ 1,728,600	$ 1,423,100
Average Fixed Pay Rate	2.10%	2.50%
Average Floating Receive Rate	2.00%	2.50%
Average Maturity (Years)	8 years 10 months 24 days	9 years 10 months 24 days
Interest Rate Swaps | Derivative Instruments Not Accounted as Hedges Under GAAP | Greater than 5 years | Variable Pay Rate
Currency swaps and forwards
Notional Amount	$ 45,000	$ 728,400
Average Fixed Pay Rate	1.90%	2.50%
Average Floating Receive Rate	2.30%	2.40%
Average Maturity (Years)	19 years 6 months	8 years 3 months 18 days
Forward Starting Interest Rate Swap
Currency swaps and forwards
Notional Amount	$ 0	$ 0