Derivative Instruments - Interest Rate Swaps (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2017
Derivative instruments not accounted as hedges under GAAP
Currency swaps and forwards
Fair Value—Asset (Liability), net	$ (3,618)	$ (161,587)
Interest rate swaps | Designated as hedging instrument | Fixed Pay Rate
Currency swaps and forwards
Notional Amount			$ 3,200,000
Interest rate swaps | Designated as hedging instrument | Variable Pay Rate
Currency swaps and forwards
Notional Amount			$ 2,000,000
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | Fixed Pay Rate
Currency swaps and forwards
Notional Amount	$ 3,252,200	$ 4,662,200
Average Fixed Pay Rate	2.20%	2.10%
Average Floating Receive Rate	1.40%	0.90%
Average Maturity (Years)	7 years 1 month 6 days	5 years 8 months 12 days
Forward starting (as a percent)	46.90%	35.70%
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | Variable Pay Rate
Currency swaps and forwards
Notional Amount		$ 2,682,400
Average Fixed Pay Rate		1.70%
Average Floating Receive Rate		0.90%
Average Maturity (Years)		6 years 6 months
Forward starting (as a percent)		0.00%
Fair Value—Asset (Liability), net		$ (42,861)
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | 1 Year or Less | Fixed Pay Rate
Currency swaps and forwards
Notional Amount		$ 105,900
Average Fixed Pay Rate		0.80%
Average Floating Receive Rate		0.80%
Average Maturity (Years)		9 months 18 days
Forward starting (as a percent)		0.00%
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | Greater than 1 Year and less than 3 years | Fixed Pay Rate
Currency swaps and forwards
Notional Amount	$ 600,000	$ 993,000
Average Fixed Pay Rate	1.60%	1.20%
Average Floating Receive Rate	1.50%	0.90%
Average Maturity (Years)	1 year 9 months 18 days	1 year 4 months 24 days
Forward starting (as a percent)	0.00%	88.10%
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | Greater than 3 years and less than 5 years | Fixed Pay Rate
Currency swaps and forwards
Notional Amount	$ 960,000	$ 1,861,700
Average Fixed Pay Rate	2.00%	1.90%
Average Floating Receive Rate	1.40%	0.90%
Average Maturity (Years)	4 years 3 months 18 days	3 years 10 months 24 days
Forward starting (as a percent)	0.00%	36.50%
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		$ 1,811,400
Average Fixed Pay Rate		1.40%
Average Floating Receive Rate		0.90%
Average Maturity (Years)		3 years 8 months 12 days
Forward starting (as a percent)		0.00%
Fair Value—Asset (Liability), net		$ (24,112)
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | Greater than 5 years | Fixed Pay Rate
Currency swaps and forwards
Notional Amount	$ 1,692,200	$ 1,701,600
Average Fixed Pay Rate	2.50%	3.10%
Average Floating Receive Rate	1.40%	0.90%
Average Maturity (Years)	10 years 6 months	10 years 6 months
Forward starting (as a percent)	90.20%	6.50%
Interest rate swaps | Derivative instruments not accounted as hedges under GAAP | Greater than 5 years | Variable Pay Rate
Currency swaps and forwards
Notional Amount		$ 871,000
Average Fixed Pay Rate		2.20%
Average Floating Receive Rate		0.90%
Average Maturity (Years)		12 years 3 months 18 days
Forward starting (as a percent)		0.00%
Fair Value—Asset (Liability), net		$ (18,749)
Forward starting interest rate swap
Currency swaps and forwards
Notional Amount	$ 1,500,000	$ 1,700,000
Derivative instrument, weighted average forward starting date	4 months	4 months 16 days