Derivative Financial Instruments - Basis Swap (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Loans receivable	$ 21,814,507	$ 24,903,724
Notes payable	21,356,573	24,668,490
One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	$ 11,250,000	$ 1,775,000
Variable interest rate spread	0.125%	0.101%
One Month to Three Month Basis Swap Outstanding 6 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	$ 4,250,000
One Month to Three Month Basis Swap Outstanding 7 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	100,000
One Month to Three Month Basis Swap Outstanding 8 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	300,000	$ 300,000
One Month to Three Month Basis Swap Outstanding 1 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	3,500,000
One Month to Three Month Basis Swap Outstanding 8 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	1,000,000
One Month to Three Month Basis Swap Outstanding 2 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	250,000
One Month to Three Month Basis Swap Outstanding 3 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	1,150,000	1,150,000
One Month to Three Month Basis Swap Oustanding 4 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	375,000
One Month to Three Month Basis Swap Outstanding 5 [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notional amount	325,000	$ 325,000
One-month LIBOR, Daily reset [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Loans receivable	20,000,000
Three-month commercial paper rate [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Loans receivable	1,100,000
Three-month treasury bill, Daily reset [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Loans receivable	600,000
Three-month LIBOR, Quarterly reset [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notes payable	11,700,000
One-month LIBOR, Monthly reset [Member] | One Month to Three Month LIBOR Basis Swap [Member]
Derivative [Line Items]
Notes payable	$ 8,600,000